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Artículo: COMEII-15025

I CONGRESO NACIONAL COMEII 2015

Reunión Anual de Riego y Drenaje

Jiutepec, Morelos, México, 23 y 24 de noviembre

EVALUACIÓN EXPERIMENTAL DE UN SISTEMA NF-PV DESTINADO PARA

APLICACIONES DE RIEGO AGRÍCOLA

Ulises Dehesa Carrasco1; José Javier Ramírez Luna2

1CONACYT Research Fellow, Instituto Mexicano de Tecnología del Agua, Paseo

Cuauhnáhuac 8532, Col. Progreso, C.P. 62550, Jiutepec, Morelos, México. 2 Instituto Mexicano de Tecnología del Agua, Paseo Cuauhnáhuac 8532, Col. Progreso, C.P.

62550, Jiutepec, Morelos, México.

Resumen

La desalinización con energía solar representa una solución atractiva para aplicaciones de

riego agrícola en zonas remotas, especialmente en cuencas endorreicas donde los

compuestos presentes, tales cómo sulfatos, exceden los límites máximos permisibles para

aplicaciones de riego. En éste trabajo se presenta un estudio experimental de un sistema de

desalinización de Nano filtración (NF) con paneles fotovoltaicos (PV) que opera sin

almacenamiento eléctrico. Con el fin de evaluar el rendimiento del sistema, diferentes

concentraciones de S04 fueron analizados. El experimento se llevó a cabo a cielo abierto

bajo diferentes condiciones de radiación solar. Con base en los resultados experimentales,

la calidad del permeado obtenido satisfice los estándares de la Ley Federal de Derechos en

Materia de Agua. Sin embargo, la concentración inicial y el ensuciamiento del sistema de

pre-tratamiento juegan un papel importante en el rendimiento del sistema. Se observó una

disminución de permeado de 7 lpm cuando la concentración inicial aumenta de 525 mg / l

a 2,539 mg / l. Esta disminución representa cerca del 2 kWh / m3 de consumo de energía. El

consumo máximo de energía probado fue 3,4 kWh / m3 con una concentración de 2,539 mg

/ l. Se observó una producción de permeado que oscila entre 2,16 a 4,08 m3 / d, por lo que

es posible irrigar entre 1 y 2 hectáreas de cultivos.

Palabras clave: Desalinización solar, riego agrícola, aplicaciones de la NF.

I Congreso Nacional COMEII 2015, Jiutepec, Morelos, México, 23 y 24 de noviembre

2

ABSTRACT

Desalination driven with solar energy represent an appealing solution for agricultural

irrigation in remote areas. In this work, an experimental study of a NF-PV desalination

system without electric storage is presented. In order to evaluate the performance of the

system, different salt concentrations (mainly SO4-2) was tested. The experiment was

conducted at open sky under varying solar radiation conditions. Based on experimental

result, the quality of permeated obtained satisfice the standards of Mexican norm for

irrigation. However, the initial concentration and fouling play an important role in the

performance of the system. Was observed a decrement of permeated of 7 lpm going from

525 mg/l to 2539 mg/l. This decrement represent close of the 2 kWh/m3 of energy

consumption. The maximum energy consumption tested was 3,4 kWh/m3 with a

concentration of 2539 mg/l. Was observed a production of 2,16 – 4,8 m3/d, making it

possible to irrigate between 1 and 2 hectares of crops.

Keywords: Solar Desalination, agricultural irrigation, applications of NF.

1. Introduction

The limited water resources are a real challenge for the actual status of agriculture in the

world. The most important drivers for water scarcity are: growing water demands as the

population increases, economic development, and the increase water per capita

consumption [1]. Experts in the field agree that the production of food locally represents a

strategic policy for undeveloped countries [2]

In Latin America, in particular the north of Mexico, exist large tracts of land that are

potentially productive for crops or pasture management. However, these lands are not

exploited because the available groundwater has not the water-quality necessary for

agriculture applications [3]. Mainly because the limit maximum permissible of some type

of salt, so as, S04-2 are exceeded by 400% as previously was reported by [4]. Without other

water sources, brackish groundwater is used directly to irrigate the crops. However, this

practice reduces the performance of agricultural production and causes a negative impact

on the soil surface by the salt deposition [2].

Desalination of brackish groundwater (BW) is an alternative that has been employed to

increase the availability of water, and indirectly reduce the negative impact of

contamination of the soil by salt. The quantity and water quality needed for irrigation is

defined depending on the crop and soil characteristics [2]. The water for irrigation is

classified based on established standards, the most important are: electrical conductivity

(EC), total dissolved solids concentration (TDS), sodium risk (as function of the sodium

absorption ratio (SAR)), the potential of hydrogen (pH) and the cations (Ca2+, Mg2+, Na+,

K+) and anions concentration (HCO32-, Cl-, SO42-, CO32-, and NO3).


3

Desalination for agriculture applications is often thought as an unprofitable process,

mainly because this is an expensive method (e.i 40-45% of the total cost) [5]. However, in

recent years this perception is changing [2],[6]-[7] . Some countries such as Spain, Israel

and United Arab Emirates have increased the volume of desalination water for agriculture

irrigation [8]. The reverse osmosis (RO) is the technology with major presence in market

desalination for large or small applications [2],[8].

Nanofiltration (NF) has been used as one part of the solution for pre-treatment in RO

desalination. The NF membrane is used to remove particles with diameters greater than 2

nm such as sulfates, which negatively affect the useful life of the RO membranes. The NF

membrane do not prevent entirely the flow of salt through the membrane; monovalent

compounds present in the mixture can cross. The presence of these compounds in

permeate flow can be beneficial for the crops, because some of this compounds are

required for the plant growth appropriately; provided that the maximum permissible

limits, stabilized in standardized norm, are not exceed [5].

An NF system works with lower pressures than RO, in consequence the specific energy

consumption is lower. This characteristics allow design and build less robust systems

where solar energy with photovoltaic panels is very attractive especially for small-scale

applications in remote areas [9].

The NF desalination process couplet with solar photovoltaic (NF-PV), has been reported

previously in literature [11],[13]. IEA-ETSAP and IRENA, publication demonstrated the

feasibility of NF-FV, for treating water in isolated places for human consumption [11]. The

results showed that this systems can be used for small scale irrigation. Richards et al.,

reported a study of desalination by an hybrid membrane configuration (NF,UF and RO).

The specific energy consumption ranged from 2 – 8 kWh per 1 m3 of disinfected and

desalinated drinking water. Ghermandi A. and Massalem R. studied the advantages of NF

membranes instead of RO membrane in the production of irrigation water. Based on the

simulation of the performance of a solar-assisted pilot plant, the energy consumption by

the proposed system was 40% lower than conventional reverse osmosis desalination,

reducing in 34% the currently abstracted groundwater volumes, and increasing in 18% the

total biomass production of the irrigated crops [5].

Recently, Jasson et al.[4], carried out an experimental study about alternative treatment

brackish water for irrigation using a NF-FV system. The study was focused in

understanding the behavior of the system, keeping constant the amount of sulfate in the

influent (1863 mg/l) which affect mainly the water quality. Authors report a production

average of 3.2 m3 / day with 6.3 solar peak hr, allowing cultivation in the region of study,

up to 15 tons of tomatoes to a rate of 35 kg / m3. Zarzo D. et al. published the Spanish

experience in desalination for agriculture applications. The authors concluded that the

desalinated water can be more expensive than water from other origins but this depends


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on many factors. However, many agricultural products can support the price of

desalinated water without a great impact on the overall price [2] [7].

Application of desalination with PV assisted as previously was described, commonly uses

a batteries support system for the storage of the photovoltaic electric energy that is on its

turn transformed in AC for powering the pumping system. However, the electricity

supplied by PV system can be used directly to energize the pump of the NF system

producing permeate only during the sunlight hours and storing the desalinated water

instead of the electric energy.

The aim of the present study is to evaluate the performance of the NF-PV desalination

system as function of exogenous variable. The electricity supplied by PV system is used

directly to energize the driven pump of the NF system, without battery support. The

system was evaluated as function of energy consumption, recovery rate and quality of the

effluent, considering the exogenous variables as irradiance and the salt concentration in

the influent. The effect of pressure drop in the system by fouling in the pretreatment filters

are discussed.

2. Materials and methods.

a) NF-PV System

In Fig. 1 the scheme of the experimental device is shown. The system is integrated by one

micro filter used for pretreatment of the influent, a stage of Nanofiltration modules (NF),

photovoltaic solar cell (PV) and pumping system. The NF stage is composed by four NF

polyamide membranes (ESNA1-LF-4040 model) connected in parallel configuration,

providing an equivalent area of 30.6 m2. The prototype was designed to operate with a

nominal capacity of permeate close to 12 l / min with a supply of brackish water of 60 l /

min. The electric supply was provided by a photovoltaic plant with a nominal power of

1.92 kW integrated by eight polycrystalline silicon modules of 240 W each module which

supplies power to a submersible centrifugal pump, model SQFlex 16 SQF-10. The coupling

of the PV plant with the pump system was direct (without battery support) and controlled

only with an on-off switch.


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Fig. 1. Schematic diagram of the NF-PV system

b) Methodology

The experiment was carried out under laboratory conditions using a solution with high

content of SO4 -2 as influent. In order to evaluate the system performance, four different

concentrations of the influent were tested. The nominal values of concentrations was 525

mg/L, 1170 mg/L, 1750 mg/L and 2539 mg/L respectively. In each test the concentration

was kept constant while solar radiation was a free parameter whit a variation along the

solar day in a range of 300-1000 w/m2. The irradiation was measured over the plane of the

PV solar cell with a Kipp & Zone pyranometer of first class with an uncertainty of ± 1.0%.

In order to calculate the power supplied by PV plant the voltage and electrical current

was directly measured with a 34972A Data Acquisition. The volumetric flow was

measured using an AQF-600-105 flow meter with resolution of 0.25 l / min and the

pressure through Ashcroft G2 pressure transducers were installed on the input and output

ports of the system.

The evaluation of the system consisted in determine the removing effectiveness of SO4-2,

the specific energy consumption, the permeate recovery rate and quality of permeate for

fertigation. The removal efficiency of SO4-2 was determined under conditions of sunny and

cloudy days with a relation of the SDT of the influent and the permeate stream.

The energy consumption was defined as the ratio of permeate flow and the electric power

supplied to the pumping system according to equation 1.

(1)


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Where represent the average permeate flow in the time as function of solar

radiation, and is the average electric power supplied to the pumping system in

the same time.

The quality of permeate for fertigation was evaluated as function of the following

parameters: EC, TDS, PH and SAR. The SAR is evaluated with the content of sodium

cations, calcium and magnesium according the equation 2, as was reported by [3],[12].

(2)

3. Analysis and discussion of results.

The experiment focused in the behavior of the NF-PV desalination unit as function of the

influent concentration and solar irradiation. Four sets of experiments were carried out for

this purpose. In each set, the concentration of influent was kept constant, varying only the

irradiance along the solar day in a range from 300 to1000 w/m2. The fixed nominal

influent concentration corresponding to the four sets were 525 mg / L, 1170 mg / L, 1750

mg / L and 2539 mg / L respectively. As mentioned above, each reported data point

corresponds to an average over 3000 measurements, during a 500 min period. The

experiment results are discussed in below.

a) Water quality

In order to quantify the water quality the pH, EC and SDT was measured. In each set, the

measurement were carrying out at the start, middle and end of the test. The concentration

remained with a maximum standard deviations of ±10 mg / L (± 19.9 µS/cm). The

experimental result is shown in table 1.


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Table 1: Evaluation of water quality

Test pH CE (µS/cm)

SDT

(mg/L) T (°C)

A 8.54 1105 525 30.8

1 B 8.55 1217 565 31

C 8.53 154 85 30.9

A 8.45 2330 1170 29.7

2 B 8.42 2630 1320 29.7

C 8.38 350 180 29.7

A 8.37 3490 1750 28

3 B 8.35 4100 2050 28

C 8.31 320 160 28

A 8.1 4885 2539 25.2

4 B 8.1 5759 3003 25.2

C 8.1 281 96 25.2

Influent (A), Effluent (B) and permeated (C)

Based on experiment observations, the "fouling” in pre-treatment filters, have an

important effect on the water quality. The suspended solids eventually can saturate the

filters. Consequently, the effective working pressure are drastically reduced as show in the

Fig, 2.

Fig, 2. Effect of fouling in the pre-treatment filter on the hydraulic head loss. A dirty filter

respect to a new one.


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A typical maintenance of the system consists in changing the pre-treatment filter with a

new one. This modification led to an increment in the inlet pressure on to NF membrane.

The NF membranes do not avoid entirely the flow of salt, monovalent compounds present

in the mixture cross through to the membrane. Water and salt have different rates of mass

transfer across to NF membrane, it allows the "rejection” phenomenon. As the working

pressure increases, the rate of water transfer increase also without changing the flow rate

of the salt. In consequence, the permeation of the salt through the membrane was lower as

is show in the experiment four (table 1).

The sulfates remotion effectiveness, based on CE measurement, can be expressed as

× 100 (3)

Where represents the amount of salts rejected and , the influent

concentration. It can be observed that the highest efficiency (94%) was obtained in the test

4th. The results are presented in Table 2.

Table 2: Efficiency of sulfate remotion

Influent

(µS/cm) Permeate Efficiency %

Test 1 1,105 154.5 86

Test 2 2,330 350 85

Test 3 3,490 320 90.8

Test 4 4,885 281 94.2

In order to quantify the amount of sulfates and chlorides, a detailed study for test 4 was

conducted. According with NMX-AA-073-SCFI-2001 (Mexican standard) were determined

chlorides and with NMX-AA-074-1981 (Mexican standard) was used for the sulfates.

Considering the NMX-AA-051-SCFI-2001 and flame method, the elements useful for the

SAR was obtained. The results are presented in Table 3.

Table 3 Determination of chlorides, sulfates and SAR

Test

chlorides

(mg/L)

sulfates

(mg/L) Ca Mg Na SAR

A 48.4 2491 344.503 220.927 53.074 0.549

4 B 52.5 2951 433.81 236.645 62.575 0.631

C 19.4 76.3 12.955 11.579 12.56 0.611


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b) Solar system

The experimental tests runs were conducted under conditions of sunny and cloudy days.

During experimental tests the voltage provided by the photovoltaic panels was kept at an

average of 217.4 volts. The graph of Fig. 3 shows a typical day of test.

Fig. 3. Solar radiation on PV level (primary axis) and volts provide by panel (secondary

axis)

Figure 4 shows the power required by the system relative to the incident solar radiation. A

linear dependence is observed. However, scattering effects were due to the radiation

dispersed by cloudiness and the effects of tilt PV system.


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Fig. 4. Power supplied to system as a function of solar radiation. The test correspond to

one day of sparsely cloudiness.

c) NF system

Permeate flow rate is affected inversely with the influent concentration. This is a

characteristic of the NF systems. Figure 5 shows the permeate production with respect to

the initial concentration. Based on the experimental results, the production of permeate

relative to the supply pressure has a linear trend. Further, the concentration inversely

affects permeate production. Observe that the increase of 525 mg / L to 2539 (mg / L)

production experienced a decline close to 7 lpm which affects the energy consumption of

the system.


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Fig. 5. Production of permeate as a function of the feed pressure.

Fig. 6 shows power consumption per unit of permeate volume. It can be observed that the

power consumption is a linear function to influent concentration: higher sulfates

concentration requires higher energy consumption. The maximum consumption was 3.4

Kwh/m3. In previous works, has been reported consumption between 0.5-4 kWh / m3 [11] .


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Figura 6 Energy consumption for deferent influent concentration

The direct coupled configuration of the NF-PV was established by irrigation requirements.

As a general rule, the crops should be irrigated when the irradiance is low, in order to

reduce losses of water by vaporization and minimize plant stress. In this context, instead

of accumulating electric energy in batteries for later use, treated water during the day can

be stored in elevated tanks. In fact, is cheaper store water in tanks that accumulate energy

in a batteries. The NF-PV system produce from of 2.16 - 4.8 m3/d, making it possible to

irrigate between 1 and 2 hectares of crops.

4. Conclusion

An experimental study of a NF-PV desalination system without electrical storage support

was presented. Experiments were carried out for different influent concentration and solar

radiation. Based on experiment observation, the quality of permeated obtained satisfice

the standards of Mexican norm for irrigation. The permeate production is affected

inversed to influent concentration. In fact, was observed a decrement of permeated of 7

lpm going from 525 mg/l to 2539 mg/l. The fouling have an important effect on the

production, was observed a hydraulic head loss maximum of 20.9 m H20. The energy

consumption is affected stronger by the initial concentration as well as fouling in the

pretreatment filters. Concern to energy consumption the maximum value was 3.4 Kwh/m3

with a concentration of 2539 mg/l. The NF-PV system produce from of 2.16 - 4.8 m3/d,

making it possible to irrigate between 1 and 2 hectares of crops.


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Acknowledgement

The authors appreciate the partial support by “1772 Cátedras CONACYT-Mexico” project.

U. Dehesa-Carrasco wishes to thanks J.J. Quiñones Aguilar and E. Delgado-Quezada for

technical support.

Reference

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Documents

I CONGRESO NACIONAL COMEII 2015 Reunión Anual …€¦ · I CONGRESO NACIONAL COMEII 2015 Reunión Anual de Riego y ... Was observed a production of 2,16 – 4,8 m ... has been used