Multi-effect distillation with heat pump integrated

BENALI ABDELHAKIM

Laboratory of ENERGY in ARID Zones, Faculty of Sciences and Technology,

Tahri Mohammed Béchar University,

Street of Independence Béchar , Bp 417,08000, ALGERIA

Abstract: This article presents a study and simulation of the desalination system consisting of a heat pump HP and a multi-effect

distillation MED unit. Electric energy using in HP is provided by photovoltaic panels and wind turbines ,for a possible installation of the

system on an isolated sites.The proposed desalination system uses an additional source of thermal energy in order to make HP and MED

integration optimal and to reduce HP Electrical energy consumption per cubic meter of distilled water (kwh/m3).The main idea is to use

geothermal-solar thermal energy and heat from HP as two thermal inputs in the multi-effect distillation unit MED. Thermal rejection from

MED is recovered to be used as heat input in HP that based on mechanical compression of working fluid. The HP can use the working

fluids (R22, ammonia) for a number of reasons, including that the two previous fluids are very dense at the saturated vapor state compared

to water. A thermodynamic analysis of the desalination system was performed at steady state, using the thermodynamic properties of the

Coolprop database. The simulation results showed a minimum value of electrical energy consumption, without consideration the

contribution of auxiliary thermal energy :(10.487 kwh/m3 | effect numbre:5).The simulation results showed a minimum value of

volumetric flow rate of the working fluid ,before compression : (17.685 m3 of working fluid per m3 of distilled water | effect numbre:12 |

contribution ratio of auxiliary thermal energy:46.6 %).

Keywords: Coolprop, thermodynamic, solar thermal, geothermal, MED, HP.

Received: June 15, 2022. Revised: August 12, 2023. Accepted: September 21, 2023. Published: October 5, 2023.

1. Introduction

Industrial and agricultural development in Third World countries

has been accompanied by intense exploitation of natural water

resources and a profound change in environmental conditions.

For a very long time and in many Third World countries, almost

irreparable damage has been inflicted on the water resources of

the stripping of the soil from its plant cover by the proliferation

of industrial activities (deforestation, grazing, etc.) [1]. Water is a

basic necessity for all living species. Its importance has increased

considerably with the increase in food requirements. Since

ancient times, man has concentrated around rivers, lakes and

ground water reservoirs to cover his water needs to meet his

domestic, agricultural and industrial needs [2]. All countries are

affected by the water problem; poor countries lack water and

developed countries pollute it. Water will become a formidable

stake during the century to come [3]. The problem of water in the

world is a problem globally not of quantity, but of quality, which

goes against the alarmist assertions of those who claim that a

time will come when the world as a whole will lack water. The

thorny problem is its distribution both within a country and

across the globe [1]. In the Asia Pacific region, where 60% of the

world's population lives, one in three people does not have access

to drinking water and the lack of water will be a factor that will

limit the production of food in these regions. . Fourteen African

countries suffer from the lack or poor quality of water and the

most optimistic studies reveal that the number of these countries

can go up to 25 in 2025, that is to say nearly half of the future

population of the black continent [4]. Finding new water

resources is a necessity to solve the problem of increasing water

demand.Sea water can be desalinated so that it can be used in

practical life, such as agriculture, drinking and industry [5].

Research on highly energy efficient desalination technologies

must therefore be particularly active.In this context,Multiple-

effect distillation or multi-effect distillation (MED) is a

distillation process often used for sea water desalination. It

consists of multiple stages or "effects". In each stage the feed

water is heated by steam in tubes, usually by spraying saline

water onto them. Some of the water evaporates, and this steam

flows into the tubes of the next stage (effect), heating and

evaporating more water. Each stage essentially reuses the energy

from the previous stage, with successively lower temperatures

and pressures after each one. There are different configurations,

such as forward-feed, backward-feed, etc. [6] Additionally,

between stages this steam uses some heat to preheat incoming

saline water. [7].

The performance of MED plants can be significantly affected by

design and operating parameters such as number of stages, top

steam temperature (heating steam temperature of the first effect),

heating steam flow rate, temperature difference in the final

condenser, etc. [8] Except for optimizing these parameters, four

types of heat pump have also been adopted for coupling with

MED to achieve higher efficiency in MED system, including

mechanical vapour compression (MVC), thermal vapour

compression (TVC), absorption heat pumps (AHP) and

adsorption heat pumps (ADHP) [9]. To some extent, these heat

pumps all aims to recover the last effect steam or the low grade

energy of it. Among them, TVC have been widely used in

commercial desalination plants since 1990s [10]. TVC features

the steam-jet ejector based on Venturi principle through which

the pressure and temperature of the steam generated in the last

effect of the MED process is elevated by introducing high

pressure motive steam. As the last effect steam is partly reused,

the required motive steam, the size of thermal energy supply

system and the final condenser are drastically reduced. In MED-

MVC, the low temperature/pressure steam from the last effect

of MED is compressed to high temperature/pressure steam

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Benali Abdelhakim

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by mechanical compressor which is mostly driven by elctric

power or diesel engine. Final condenser is eliminated in this

configuration which make it even more compact. However,

presently MVC application is limited to small to medium

desalination plants with maxmum 5000 m3 /d capacity [11]. One

of the main advantages of heat pump system is that a large

amount heat added or removed versus natural process of heat

transfer by temperature difference. This process is performed by

doing work on working fluids (almost refrigerant). This work

almost is several times lower than the quantity of heat removed

or added. That means the quantity of heat removed or added by

the heat pump system is not actually considered as a cost

indicator but the work done through the compressor is the

indicator. It is well known that the thermal distillation process of

sea water to produce fresh water consumes a large quantity of

energy in the form of heat [12] [13]. The combination of heat

pump and distillation systems to desalinate seawater may be

promising and simple economic method compared with

traditional desalination techniques [13]. Also, improving heat

pump performance is in the context, [14] introduced a various

methods of enhancing the performance of heat pumps which

followed by a review of major hybrid heat pump systems suitable

for application with various heat sources.

The main idea of our research paper is to study the MED with

HP that based on mechanical compression of the working fluid

using electricity from photovoltaic and wind energy. The heat

pump HP is improved by auxiliary energy, in order to make HP -

MED integration optimal and to reduce HP Electric energy

consumption per cubic meter of distilled water (kwh/m3).The

proposed HP Recycle the rejection heat from MED unit, in order

to use this heat again in seawater desalination. The usual working

fluid (water vapor) used in MVC-MED installation was replaced

in proposed MED-HP by dense working fluids at saturated vapor

state (R22, Ammonia). Consequently, dense working fluid

reduces the volumetric flow rate to be compressed.

2. Description and method of study

The description contains an explanation of the working principle

of HP and MED and the relation between them. A study method

describing the content of the thermodynamic calculation, such as

the sequence of arithmetic operations and classification of

parameters categories in calculation,etc.interpretation as follow :

- Figure.1: the multi-effect distillation MED units make use of

the vapors evolved in the first evaporator by condensing them in

the second evaporator. The heat of condensation is used to boil

the seawater in the second evaporator. Likewise, the third

evaporator acts as a condenser for the vapors of the second

evaporator, this process is repeated up to the final evaporator,

and each evaporator in this chain is called the effect. Each effect

has a pressure slightly higher than the next effect, causing the

temperature difference between condensation and evaporation in

each effect. Sea water is chemically treated and then sprayed

inside each effect. The brine flows naturally from the effect to

the next effect without pumping, due to the pressure difference.

- Figure.1: working fluid is heated to become a saturated steam

by an HP recovery device that using the condensation energy of

the vapor from the last effect ; The saturated vapor is compressed

at a pressure having a higher condensation temperature than the

first effect temperature ; The vapor condenses in HP condenser

located inside the first effect,producing thermal energy to boil the

sea water. Finally, the liquid discharge from the HP condenser is

expand at a pressure having an evaporation temperature lower

than the last effect temperature , to new cycle. Auxiliary energy

is activated by using additional heat input in the first effect.

- The impact of 2 parameters « effect nombre n | contribution

ratio of auxiliary thermal energy Ri » on the operation of the

desalination system was evaluated. In order to assess this impact,

a thermodynamic calculation based on standard manufacturer

parameters such as ηis,c , ηmo ,etc and based on actual operational

parameters such as Tn, Re,etc. Thermodynamic calculation begins

with the MED unit cycle and then proceeds to the heat pump

cycle. The output parameters for each effect are used as input

parameters for the next effect. This thermodynamic calculation is

repeated with a change in the values of the parameter studied

«n».The purpose of this thermodynamic calculation is to

determine all the output operating parameters such as :

 Volumetric flow rate of total distilled water.

 Volumetric flow rate of working fluid.

 Compression ratio of compressor.

 Electric Energy consumption.

 Performance Coefficient.

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Figure .1: Multi-effect distillation with heat pump integrated , MED-HP.

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3 Thermodynamic analysis

3.1 Hypotheses

 MED unit and HP are perfectly insulated.

 Transformation (MED Cycle: evaporation and condensation |

HP Cycle: 1 to 2 and 3 to 4) occurs at constant pressure.

 The impact of vacuum on the mass balance in each effect is

considered negligible.

3.2 MED Cycle

ṁdistilled(i=1) = ṁsea.Re , ṁbrine(i=1) = ṁsea – ṁdistilled(i=1) ,

Sbrine(i=1) = Ssea / (1-Re) .

mdistilled(i) = [ ṁbrine(i-1).Cbrine.(Ti-1–Ti) + ṁdistilled(i-1).Hv(i-1)

– ṁsea.Csea.(Ti–Tamb) ] / Hv(i) , Cbrine ≈ Csea ,

ṁbrine(i) = mbrine(i-1) + ṁsea – mdistilled(i) ,

Sbrine(i)= [Sbrine(i-1).ṁbrine(i-1) + Ssea.(ṁsea / (ṁsea – ṁdistilled(i)))

.(ṁsea – ṁdistilled(i))] / (ṁbrine(i-1) + (ṁsea – ṁdistilled(i))) .

󰑂󰇛󰇜  󰑂󰇛󰇜



 󰇛󰇜

3.3 HP Cycle

ṁdistilled(n).Hv(n) = (1-Tv).ṁwf.Hv(wf) = Qe

ṁwf.h1 + Qe = ṁwf.h2 , v

wf(2) = 3600.ṁwf / ρwf,(2)

ṁwf.h2 + W (2-3)r.ṁwf = ṁwf.h3 , W(2-3)r = W (2-3)is / ηis,c

ṁwf.h3 – Qc = ṁwf.h4 , ṁwf.h4 = ṁwf.h1 .

COP = QCondenser / ( W (2-3)r.ṁwf ) , P = W (2-3)r.ṁwf / ηmo ,

QCondenser + QAuxiliary = ṁsea.Csea.(T1 –Tamb) + ṁdistilled(1).Hv(1)

, Ri = QAuxiliary /( QCondenser + QAuxiliary).

3.4 Input parameters

Table.1 : Input parameters of MED and HP , properties.

Parameters ,properties at 25 °C

symbol

value

Seawater salinity

Ssea

34 kg/m3

Seawater density

ρsea

1022.6 kg/m3

Specific heat of seawater

Csea

4002 J/kg.°K

Seawater flow in each effect

ṁsea

200 m3/hour

Density of distilled water

ρdistilled

997 kg/m3

Rate of evaporation in effect N°:1

0.5 %

HP : Condenser pinch point

Tc – Ti=1

3 °C

Effect number

HP : Evaporator pinch point

Tn – Te

3 °C

Temperature betwin two Effect

Ti-1 – Ti

2.5 °C

Compressor isentropic efficiency

ηis,c

80 %

Motor efficiency

ηmo

98 %

Temperature of the last effect

35 C °

4. Results and interpretation

Table.2 : Output parameters from MED.

(°C)

(bar)

ṁdistilled

(kg/s)

ṁbine

(kg/s)

Sbrine

(kg/m3)

(kj/kg)

52.5

0.139

28.405

2375.9

0.123

26.066

59.148

65.311

2381.91

47.5

0.108

24.105

91.852

63.085

2387.98

0.095

22.528

126.133

61.252

2393.99

42.5

0.084

21.339

161.603

59.759

2399.99

0.073

20.54

197.87

58.566

2405.98

37.5

0.064

20.131

234.548

57.643

2411.95

0.056

20.111

271.247

56.965

2417.9

( ṁdistilled(tot) = 661.57 m3 / hour)

Table.3 : Output parameters from HP .

State

(°C)

(bar)

(kg /m3)

(%)

(kj/kg)

(kj/kg-k)

Ammonia

12.38

81.46

0.1

468.67

1.90

12.38

9.59

1487.11

5.24

86.31

23.4

15.35

1596.97

5.3

55.5

23.4

553.32

468.67

1.88

R22

12.55

244.13

0.18

293.13

1.35

12.55

53.52

436.80

1.82

67.98

88.86

453.51

1.83

55.5

1054.6

293.13

1.34

COP

P /ṁd(tot)

(kwh/ m3)

wf(2) /ṁd(tot)

mwf

(kg/s)

R22

1.752

9.6

0.265

8.531

34.347

337.848

Ammonia

1.89

10.27

0.272

7.9

47.575

Figure .2: compression ratio of compressor.

Figure .3: performance coifficent.

1,1

1,3

1,5

1,7

1,9

2,1

2,3

2,5

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

Rp(p3 / p2)

Effect number (n)

R22

Ammonia

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

COP

Effect number (n)

R22

Ammonia

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Figure .4: contribution ratio of auxiliary thermal energy.

Figure .5: Electrical energy consumption per m3 of distillated water

without consediration auxiliary thermal energy contribution .

Figure .6: Electrical energy consumption per m3 of distillated water.

Figure .7: Volumetric flow rate of warking fluid before compression, per

m3 of distillated water.

Fig.5,6: The electrical energy consommation increases due to the

low effect number . the electrical consommation has minimal

optimal value when effect nombre between 4 and 7. The results

showed that the increasing in electrical energy consommation in

range of (n=7;12) can be easily eliminated by using the auxiliary

energy contribution. This means that using auxiliary energy can

improve the HP efficency. Fig.4 and Table .2: concluded that the

increasing in effect number can reduces the amount of vapor

condensed in the last effect, and consequently, the amount of

working fluid and HP condenser power decreases respectively,→

the system needs a greater contribution of auxiliary energy in

order to maintain the rate of evaporation in the first effect

( mdistilled(1) / msea=0.5 ). Fig.2 to 7: The results of ammonia and

R22 are very equal, with a slight advantage to ammonia. Fig.7,2 :

the correlation between reduction ( v



wf(2) / ṁdistilled(tot) ) using (n)

number and energy conservation of desalination ,→ require an

increasing in compression ratio Rp , and thus an increase in work

of compression per cubic meter of working fluid. (COP) ~ (n-1) is

logical , because the condenser temperature in HP (Tc) increases

due to the increase in effect number (n).

5. Conclusion

Finally , The MED-HP desalination system is powered by 100%

renewable energy, making our system sustainable with zero

carbon emissions.All the input parameters (especially the studied

parameter: n) , all the energy and mass balances, laws were used

in simulation of the MED-HP system at steady state. Based on

the results obtained from the simulation, the effect nombre (n)

and the contribution ratio of auxiliary thermal energy (Ri) could

have a significant impact on optimization of MED-HP system.

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Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

The author contributed in the present research, at all

stages from the formulation of the problem to the

final findings and solution.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

No funding was received for conducting this study.

Conflict of Interest

The author has no conflict of interest to declare that

is relevant to the content of this article.

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(Attribution 4.0 International, CC BY 4.0)

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