DesulMn, 32 (1980) 239-256 8 E&vier scientifir Publi&ing cQmpRny* Amsterdam-RWedinTheNet&e&n~ DESIGU ASPECTS OF A SOLAR ASSISTED REVERSE OSUUSIS DESALTIHG WIT FOR URDAU COMiUllITIES

ALI H. EL-NASHAR ARD A. A. HUSSEIriY

Hechanical Engineering Department, Florida Institute of Technology, Melbourne, Florida and Science Applications, Inc., Ames, Iowa (USA); Science Applications, Inc., Ames. Iowa (USA)

SUIWRY

A conceptual design of a solar-assisted reverse osnosis (RO) plant is de-

veloped wherein the necessary power is provided by the combination action of a

diesel engine and a solar thermal energy driven Sterling engine. The design

parameters for this desalting plant are identified and a preliminary economic

analysis is made.

For urban comunities which are located in arid areas with favorable solar

insolation and climatic conditions. potable water supply could be provided by

desalting brackish water - which exists in most locations using the energy ef-

ficient reverse osnosis (RO) process and utilizing solar energy. The use of

solar energy for producing potable water from brackish water sources- using the

R@ nembrane technology is discussed in this paper. The so?ar energy could as-

sist or replace the depleting fossil fuels.

Since the RO desalting process utilizes nechanical or electrical power for

its operation, the solar system should be able to convert the thermal solar

energy into mechanical or electrical poker. This could be conveniently carried

out usin? one of the thermodynamic cycles develope@long ago for converting

thermal energy into nechanical power. Several cycles have been proposed in con-

nection with solar applications, namely Stirling, Rankine, and Brayton. The

cyc?e with the potentially highest efficiency seems to be the Stirling cycle.

The purpose of this paper is to present the main design paraneters and

economic estinates of a solar-assisted RO system using a Diesel erqine as the

non-solar ener9y source and a Stirling engine for the solar energy source.

SYSTEH DESCRIPTIUD

The block diagram of the s&ten used in this study is shown in Fig. 1. The

system is composed of a solar collector-Stirling engine subsyste;;l, solarthermal

storage, Diesel back-up unit, electrical generator and the RO unit. Part of the 239

240 EL4WXA.R AND HUSSEINY

collected themal energy during sunshine periods can be used to operate the

Stirling engine and the other Fart can be stored in a high-temperature storage

vessel. The stored energy can be used to operate t,. Stirling engine during a

part or all of the no-sunshine period - depending on the system design. Aback- up Diesel engine is to be used to provide the power required by the R-0. unit

when no solar energy is available. Either the Stirling or Diesel engine can be

in operation at any time.

Solar Through L_ Storage

Generator - R*oa - Unit

Diesel , Backup

7 - Fresh Water so+lr Stirling R-0.

Collectors Engine Unit

1 Concentrate

I

High Temp,

Diesel - Fossil

Storage Engine Fuel Feed

Fig. 1. Solar/Diesel hybrid system

The mechanical power generated by the Stirling and Diesel engines are used

to drive an electrical generator which in turn provides the pumping power re-

quired by the R-0. unit.

EL-NASEAR AND HUSSEINY 241

The solar collector

The solar collector used is of the stylized paraboloidal reflection concen-

trator type as shown in Fig. 2. The cavity receiver contains an openinj through

which the ref?ected solar rays enter the cavity formed behind the opening. A

sodiun heat pipe is used to transfer the themal energy from the cavity to the

Stirling engine. The net useful heat generated is assumed to be pumped without

loss via the heat pipe fluid to the engine.

/ /

J /

Thermal

Storage

l Output Work to Generator

The thermal stora9e

The high temperature heat store is an insulated tank filled with a cateria?

suitable for absorbing and releasing large amounts of heat (at the proper tenp-

ature range) in a nininun volune or mass. The storage capacity is dictated by

the design approach. The storage material used is 72% HaF/25% MgF? eutectic

salt nixture which has a latent heat of fusion of 0.353 kWh/kg [l]: This mix-

ture is suitable for a Stirling engine with an operatinp tenperature range be-

tween 550°C and L?OO°C.

The Sti rl inn Enai ne

In depth review of the state-of-the-art of Stirling cycle enSines are

given in [2-41. The older style single cylinder, single acting, two-piston

rhombic drive engine is typified in the Philips l-98 engine and its derivatives.

About 30-40 of these engines have been built by Philips and United Stirling of

Sweden [S] and it has been proven to be a highly reliable development engine. Max-

inun power output for these enSines were 25 Sp at 3500 rpn, heater tube wall

tenperature of 700°C, coolin wall inlet temperature of 15’C, and with hydrogen

working gas at a mean pressure of 3100 psi. Various engines have been built up

since the development of the l-98 engine, including the 4-235 engines with

power output 100-200 hp. One of these units is installed experimentally in a

OAF bus [5].

242 ET.FNkQ%HAR AND EUSSEINP

The ideal the~odyn~~ic cycle of these engines operating between a high temp-

erature TH and a law terrperature TL is shown in Fig. 3. In this cycle both the

heat added ?in and the heat rejected gout take place at constant temperature and

hence the maximun possible thermodynamic efficiency can be attained.

1-2 Qin: T - constant - TH- 2-3 V = constant; T decreases.

I -- 1-A State 4 3-4 P out: T - constant = TL.

4-1 v = constant; T increases,

Fiq. 3. The Stirling themodynarnic cycle.

The R-O_ rrnit

The R-0. process for water desalination uses specially prepared membranes

which permit the flow of water through the membrane ihhife partially rejects the

passaTe of salts dissolved in the solution. These i;ezzbranes and their supporting

itraterial is contained in specially designed pressure vessels called “modules”. A

basic flow diaTrar; of the RO process is shown in Fig. 4. The feedwater (brackish

xater in our case) is usually filtered and pretreated for the purpose of prolonging

the effective life of the peh*branes. The feedrrater is raised to the operating pres-

sure ty the high pressure pmp and nade to flow through the LO. modules. A portion

of the feed rtrean diffuses through the me!abranes as product water which flow

through a collection system to the product pump.

Meanwhile that portion of the feeditster stream;; which does not pass through the

r;.ernbrane beco;res increasingly rore concentrated with salt. The pressurized reject

brine Fay be discharged direct’ly or it may be made to pass through a turbine-genera-

tor system for recovery of some of the energy. For 2.0. units with capacities of

lOQ,OOO gpd or larger, a turbine-Generator recovery system will be considered_

R .ii. units Mtf: capacity ran@ng fro3 5,000 gpd to 1 t:! gpd ~511 be considered in this

analysis.

243

Discharge to waste

Pretreat?nent

1'

High Pressure Process ZbnIp

RO Modules

_J Brine

I ti

Product Stream

Product 4 -P

Product Water

Storage Tank

Fig. 4. R-0, process flow diagram,

PRELINIW.RY SYSTEM AW_YSIS

PokTer requirement bv R-0. unit

The solar-assisted R-0. plant is assumed to operate at full capacity for 24

hours per day with a plant factor of 0.9 (328 days per year). The energy conruned

by the R.O. process will be mainly used to drive the following pumps:

a) High-pressure pump,

b) Feed water pump,

cf Reject brine (or concentrate) pump.

d) Product water pump,

e) Miscellaneous pumps for chemical injection, backwash, etc.

The totaf specific energy requirement per kilogaallon (k gal) of product is the

sum of the products of the specific energy requirement of tSe individual pumps and

their respective relative flow rates; that is

=P _ =iT, (SE)% (RF)i k!4h/k gal

244

where SEp =

RF =

n =

EtNAsEAR AND EUSSEINY

total specific energy requirement for pumps, kWh/kgal of product water,

relative flow rate, and

number of pumps.

Table 1 lists the relative flow rate in ten% of the product water recovery

Y. Also, given is the total driving pressure, P in psi for the different pumps.

TABLE 1

Flow rate and driving pressure for pumps

ratio,

Pump Type Relative Flow Rate (RF) Total Driving Pressure “P”(psi )

Feed water puinp 1 V 60

Product water pump 1 80

Reject brine pump 1 30 -- Y1

High-pressure pump 1 v

400 - 600 (brackish water)

The specific energy requirement for each pump [63 is given by:

(6.96 x 10-3) (P) SE = kGlh/ kgal

nP

where P = total driving pressure, psi

l-JP = total pump/driver efficiency

The ener@gy available for recovery from the reject brine can be estimated using

the relation:

SEr = 6.36 x 10m3 (P - AP) nt (+ - 1) kVh/ kgal

where SEr = specific energy recovery, kWh/kgal,

AP = fluid pressure loss through R-C?. unit, psi, and

% = efficiency of recovery turbine/generator.

The net plant specific energy requirements is then given by:

SEnet = SE P

- SE, kNh/kgal

Table 2 gives estimates of typical R.0. plant net specific energy requirement for

various recovery ratios for brackish water feed.

TABLE 2

Estimates of typical RO plant specific energy requirements (kilh/kgal )

0.5 0.6 0.9 No Energy Recovery

.

0.5 0.6 0.9 Energy Recovery

High pressure

Feeduater

Product

Reject brine

Other

Ener9y Recovery (-)

TOTAL (kWh/kgal )

7.40 6.17 4.12 7.40 6.17 4.12

1.12 0.93 0.64 l-12 0.93 @.64

O-73 O-73 0.73 0.73 0.73 0.73

0.25 0.19 o-(34 O-28 o-19 O-04

0.30 0.30 0.30 0.30 3.30 0.30 __-_ ---- ___- 3.10 2.05 0.35

9.83 8.33 5.83 6.73 6.28 5-48

Assumptions :

Pump/driver efficieny = 0.72

High pressure pump with 400 psi outlet pressure

Recovery turbine/generator efficieny 0.65

The values of the design parameters used for the R-0. unit are as follows:

1. Recovery = 90% of feed flow

2. Operating Pressure = 4OD psig

3. Feed Mater Salinity = 2000 ppm TDS

4. Recovery Turbines Installed for Plants with Capacity Greater than 100,ODO gpd

5. Recovery Turbine/Generator Efficiency = 0.65

6. Pump/Driver Efficiency = 0.72

7. Plant Factor = 0.9.

The solar collector-the-1 storage-Stirling enqine system

The fraction of the incident solar radiation absorbed by the heat pipe fluid can

be expressed by the collector/receiver efficiency nc,.

246

where gin = rate of flow of thermal energy into heat pipe fluid, k’rl(th).

Ac = collector area, m2,

Is = incidence solar inadiance per unit wave len9t.h kK/q2 &XI,

I = solar insolation W/E?, and

x = wave length, JIIX

The value of qcr depends on: a) the concentration ratio “CR” of the concentrat-

in9 collector, and b) receiver temperature. Figure 5 shows the effect of these two

paraneters on the collector/receiver efficiency and the Stirling engine efficiency

for a similar system [S]. The themal efficiency values for the Stirling engine

n St3

are typical values measured during engine tests carried out using average engine

Coolant tmperature of 35’C [5]. Figure 5 also shows the system efficiency qs de-

fined as ns = rlCr t-~s~c. J

The operating temperature ran9e of the solar systm will be

from 55O’C to 9OO’C.

A paraboloidal concentrator having a value of CIZ = 1000 was selected for this

study based OR the data of Fig. 5. Three values of solar insolation I are used in the analysis; nanely 1=200, 250,

and 300 Ll/ri?. These values represent locations with average to high solar insolation.

These values of I are daily averages of the sclar insolation.

70,

a4 0 200 400 600 800 1000 1200

En&e/RCVR TempeaWze, %

Fig. 5. ccmponent efficiencies

EL-NASAE Am EusSEINP* 247

ECOROWIC ESTI:iATES

Host of the cost data for the solar system and Diesel unit used in this analysis

are derived from Ref [7] . The cost data for the R-0. unit are based on Ref [a] with

adjustments made based on the Engineering Hews Record Building Cost Index to ob-

tain approximate estimates of current costs [9]_ The economic parameters used in

this study are shown below.

The capital costs for the R-0. unit, include all process mdules, chemical storage, piping, and instrmintation. The costs associated with land purchase, land

development) and brine disposal are not included.

Cost parameters

l-

2-

3-

4-

5-

6-

7-

8-

9-

10 -

11 -

cost of parabolic dish collector = $211/m2

Sfirling Engine and Heat Pipe (assumed in mass production-See Fig.6) cost of = $180/nu~ej

Cost of Diesel Engine = $160 - $22O/kW(c) depending on engine power

Cost of storage (72% ?!aF/25% f4gF2) = $38/kWh(th)

Operation and maintenance (excluding eneraJ cost for the Diesel engine) costs for both the Stirling and Diesel engines is calculated as follows:

Annual 0 & t1 cost = (initial installed engine cost) x (0.3) x (engine load factor)

Current cost of Diesel fuel assumed $2.66/106Btu.

Xarket rate of annual fuel cost increase = 10%

Loan interest rate = 9%

Insurance = 1%

Capital recovery period = 10 years

Plant 1 ife time = 2D years

It is assumed that Stirling engine/generator sets

sites for approximately $18O{kU, if a large market for

The operating costs of these units are computed on the

can be mass-produced in small

small unit develops [7], Fig. 6.

basis of one overhaul each

8,760 hours. It is assumed that overhaul costs are 30% of the initial cost of the

engine (6 mills/kWh for an engine costing $laO/kW).

The annual cost of the solar-assisted R.O. plant can be made up of: a) the fixed

charges on the capital cost, b) the operation and maintenance expenses (excluding

enera costs), and c) cost of fossil fuel consumed.

1 I ,

200 400 600 800 3000

l

---

Installed cost of Diesel generator sets (building cost Eile)

Assumed cost of Stirlag engines

in limited production

Assumed cost of Stirling engines

in mass production

( All costs include generators.

controls, switch gear, fuel tanks,

E installation )

Genemtor size CkWel

Fit. 6. Cost of Stir1 ing and Diesel engines/generator sets

Fuel savings

The initial annual fuel cost Cf can be expressed by the relation:

Cf = 71.55 (l-s) !“RO

‘Dies.%

where s= 1.1 ..” ,n =

nDies .=

nC =

!Gth an

fraction of RD power requirement provided by solar

porter requirenent for the RO unit, M;(e)

thermal efficiency of Diesel engine-assumed 0.338

generator efficiency-assumed 0.95

assumed plant 1 ife time of 23 years and an annual fuel price escalation

rate j, the life-time saving in fuel expenses can be estimated. If S1, S2, . . . . S,

are the annual fuel savings during the lst, Znd, . . . . and nth year of plant life (in

our case n=ZO), then:

S = 71.55 “RO s 1

nDies nG

S 2

= 71.55 !‘RO Zs(l+j)

n!Jies nG

s = 71.55 (%O) s (l+j)"'l n

nDies nC

Ebl?AsEAR AND HUSSEINP 249

The total life-time saving ST = S1+S2+S3+ .__ + Su

= 71.55 ‘ROs

%ies % I+(l+j)c(l+j)2~__.~(ltj)“-1

Justifiable additional capital expenditure

Connected with the above mntioned savings in fuel expenditure due to solar

energy use, there is a justifiable capital expenses “A” attached with the solar

system This can be calculated from [lo]

A = sl (&ff)” - i ieff (l+ieffln

where A = initial justifiable capital investment. $

SI= initial annual fuel cost saving, $

n = no. of years

where i = annual interest rate j = rate of fuel price escalation

Results The percentage of the R.O. power requirement to be provided by solar versus

collector area is shown in Fig. 7 for the three R-0. capacities, 5000; 10,OgO; and 100,000 gpd and for three solar insolations I = 200, 250. and 300 N/m*. As ex-

pected, the larger the collector area and solar insolation, the higher the percentage

of power by solar. The collector area generally provides themal energy for the

Stirling engine and to the themal‘ storage tank. Uith 100 percent of power generated

by solar, the Stirling engine is run directly by the solar collector during most of the sunshine period and is run from thermal storage during the rest of the 24 hour period.

The effect of the storage period (the period during which the Stirling engine is

operated from themal storage) on the plant capital cost in $/gpd is-shown in Fig. 8 for three plant capacities: 5,000 gpd, 100,OgO gpd, and i million gpd. Increasing

the storage period causes an increase in capital cost due to the increase in the

collector area and thermal storage capacity. The capital cost decreases for higher

solar insolation because of possible reduction in collector area, see Fig. 9.

250 EL-HbsHbR AND EUSSEINP

loo- so-

80- 70-

60-

50-

I - 300 u/m2 I - 250 u/m2 ,I-

R.O. Capacity -100.000 gpd

I - 300 ujnl2 I = 250 - 100 - 4 , wha2/

90 - BO-

70- 60 -

50 -

R.O. Capacity = 10.000 gpd

200 uim2

~-_~__~--_L_-_~_- -L- _---

I = 300 . wfrn2 I - 250 wh2 I =

100 I

90 -

80-

70-

60-

R.O. Capacity = 5,000 gpd

200 Wlrrr2

AND EUSSEINY 251

Ng, 9, Capital cmt versus plant

252 EL-NAsw AND HDSSEZHP

The capital cost attributed to the solar equipment in $/gpd versus solar frac- tion for two plant capacities l!l,@OO gpd and 1 f-! gpd is shown in Fig. 10. Also

shown is the ‘life-tine savings in fuel expenses for both plants expressed in Q/gpd, For the 1 tl gpd plant, the life-tine savings are substantially higher tal costs attributed to solar equipmnt. However, for the 10,000 gpd

not the case.

than the capi- plant this is

Life-Time Fuel.

I - 200 wrn12 _..--

= 250 WQ - soar/a2

lar CapitdL Coetr<Sfgpd)

R-0. PLANZ CAPACXTX = 10,000 @

A----

3.0 PLANT CAFAcITX - 11 Spd

_/= .I

z*o- _/-'

Life-Time Fuel Savings _I'- +I“

Sohr lhsolation

I - 200 u/in2 = 250 w/ar2

I,O- = 300 Wfin2

Solar Capital Costs (Sfgpd)

Fig_ 10, Solar capital cost and life-time fuel savings versus solar fraction

EbNASHAR AND TilXlSEINY 253

Table 3 gives the solar capital CQSt (capital attributed to solar) and the jus-

tifiable initial inveskent for plants with capacities of 5,000; 10,000; 100,000; and

1 ?i gpd for different solar fractions. The justifiable initial investment is higher

than the solar capital cost for s=D.5 and is lower for S=l.

TABLE 3

Solar capital cosi?and justifiable initial investment for solar-assisted R.O. systeiz

Plant Capacity Solar Fraction

(gpd) x ID3 S

5 0 0 0 I, 0.5 2.242 2.351 II 1.0 5.672 4.703

10 I; z5 II Lo

100 II 00.5 I, 1.0

1000 I, 0”s It 1:D

Solar Capital Cost Justifiable Initial Investient

lo3 4 lo3 S

40 441 11:386

i-703 9.406

4: 159 115: 221

46; 021 1163: 686

0 47.592 95.104

0 475.777 951.554

* Solar caoital cost = cost of solar sollectors c cost of thermal storage f cost of Stir1 ing’ engine.

The C&!! expenses (excluding energy cost) are shown in Fig. 11 and the ener9y

cost is shorn in Fig. 12 for different values of solar fraction. By increasing the

solar fraction the OEi!i costs slightly increase due to the increase in collector area,

and thermal storage capacity which in turn means higher solar capital and higher

solar O&I! costs. As we expect, the energy cost decreases with increasing the solar

fraction and is zero for S-l.

The water cost is S&al is shown in Table 4 for all plant capacities studied

and for different solar insolations and solar fractions. The cost of water rises

steadily with increasing solar fraction. The lowest water cost is for an R-0. unit

where the Diesel engine provides 100% of the load. llith 50% of the power supplied by

solar the cost of water increases from 13% for the lowest capacity (5000 gpd) to 23%

for the highest capacity (1 !<gpd) when average daily solar insolation is 300 D//in2.

254

X.0. FLANT CAPACITY - 100,CMlO gpd

R-0. PLANT CAPACITY - 5,000 gpa &

- 5-

Fig.33.. operaticmand~enan~ -<-=lW

energyaxt)veras zb.rfIaction

Plant capacity 5,000 gpd to 1 LI gpd

3.00 $flO= Bta

A - Sofivt F&n 06 R-0. Pow&

Fig.12. BleI?gaxtofviater- solarfr;acticm

EJL-NAsE4R AND EUSSEINP 255

TASLE 4

Uater cost from solar-assisted R-0. plants

Plant Capacity Solar Insolation

103 9pd W/m*

Solar Fraction

S

Hater Cost

S/kgal

5 -me 0 1.21 ,-___-__-___-__-_---.______------------

!i 200 0.5 1.45 II 250 II 1.40 ,I 300 I‘ 1.37

,________,___--_-----.___-_----_________ 5 200 1.0 1.83 11 250 II 1.73 II 300 V 1.67

10 ___ 0 1.18 ~-------------------.-__-_-_--__---_-_-

10 200 0.5 1.44 I, 250 II 1.39 II 300 11 1.35

----___-._-_-______-----------.-------- 10 200 1.0 1.83 li 250 II 1.72 11 300 I, 1.66

100 ___ 0 1.13 .------_-.--__-_-~-_~---------_-_-_-___

100 200 0.5 1.40 II 250 II 1.34 II 300 II 1.31

.----_-_-_-____-----._-----___._----___ 100 200 1.0 1.78 ,I 250 II 1.68 (1 300 I! 1.61

l!IO!Il mm_ 0 0.81 _-------_.---------------------0----____

1000 200 0.5 1.10 II 250 II 1.05 II 300 ,, 1.02

.--~-_---.---~-_-___~--------____-_____

1000 200 1.3 1.49 II 250 ,I 1.39 ,I 3co 1, 1.32

256 EL-NASHAR AND HUBSEINP

Cc;ICLUSI3I~S

The followino conclusions can be made based on this study:

a)

b)

Based on optimistic Stirling engine costs andrealistixcollector and storage

costs ) the economic feasibility of solar-assisted RO plant is not favorable

at the present tine.

In arid areas of the world with a hi9h solar insolation and where fossil

fuel is not easily accessible or can be obtained for a very high price,

either solar-assisted or solar powered RO plants can be build to provide

essential water suppl ies.

REFEREHCES 1 _.

2.

::

5.

6.

7.

8.

9.

Boer, K.N., Ed., “Sharing the Sun-Sc?ar Technology in the Seventies”, Vol. 5, p. 85, Joint Conference, American Section, ISES and SESC, Aug. 1976, Published by Ar.erican Section, ISES. Postma, ?4.D.; Van Giessel, R.; and Reinink, F., “The Sti rl i nq Engine for Passenger Car Appl i cations”, SAE Paper 730648, June 1973. l?eijer, R.J., ‘*The Philips Stirling Engine”, Inqenieur, Vol. 81, i!os 18,19 (1969) Van Beukerino, H-C., and Fokker, tl., Stirling Engine”, SAE PaFer, 1973.

“Present State-of-the-Art of Philips

Pons, R-L., and Fox, R-J.. "The Solar Stirlinc Total Energy System", p. 77, Vol. 5, Proceedings Joint Conference Anerican Sectron ISES and SESC. Aug. 197G. Curran, if.l!., Oykstra, !I. j and Kenkermath. B_ , “State-of-the-Art of Pienbrane and Ion Exchange Desalting Trocesses”, MRT Reaort I!o. !!I-658, :lov. 1976_ Office of Technoio9y Assessment, Congress of the USA , “Application of Solar Technology to Today’s Enerrjy Xeeds” , Vol. II, Sept. 1978. Reed, S.A. * and Savage, H.F., “The Impact of Increased Fuel Costs and Inflation on the Cost of Desalting Sea Mater and Brackish Naters” , R!JSIA Journal, Vol. -3, 80. 2, July 1976, P-11. Engineerinq :!ews Record, Vol. 197, Ro. 5, !*?cGraw Hill Inc., New York, RY, July 29, 1976, p. 30.