1

Use of Concentrated Solar Thermal Energy Systems to Enhance Sea Salt Production in Southern Spain

Robert Borja Flagg Díaz

The Pennsylvania State University

University Park, PA 16802 USA

rbf5028@psu.edu

Susan W. Stewart, Ph.D.

The Pennsylvania State University

University Park, PA 16802 USA

SStewart@psu.edu

Jeffrey R.S. Brownson, Ph.D.

The Pennsylvania State University

University Park, PA 16802 USA

nanomech@psu.edu

ABSTRACT

This study describes a simulation analysis to asses

increased productivity of salterns used for sea salt

production by implementing a solar parabolic trough

collector to accelerate the evaporation process. The

proposed system is designed for Southern Spain, to be

coupled with a heat exchanger submerged in the saltern’s

channel. A thermal-fluid analysis of the proposed system

was modeled using Engineering Equation Solver (EES) to

replicate the heat and mass transfer behavior of the system. Results provided through the simulation model show that

the added heat increases the pond solution layer

temperature, thereby increasing the evaporation rate and

the sea salt production rate.

1. INTRODUCTION

This work proposes implementing parabolic trough

collector technology to modernize the traditional salterns

of the Bay of Cádiz, in southern Spain. These salterns,

locally known as Las Salinas, are dedicated to producing salt mainly composed of NaCl. Unfortunately it is a dying

industry which cannot meet the competitive production

rates mandated by industrial and commercial competition.

The salinas, typically small-family businesses with limited

economic resources, have been conservative about

implementing technology. The traditional process currently

employed depends largely on daily temperature and the

prevailing winds to promote evaporation. This passive and

simple system has been used throughout the centuries,

dating back to the Phoenician time periods, with little

advances [1].

As a result, the production period is limited by the

meteorological conditions that promote an adequate

psychrometric balance, which only occur during the

summer months, typically between June and October. The

ideal meteorological conditions mainly consist of daily

high temperatures, partial pressure of water vapor in air,

and easterly winds. These eastern winds, known as el

viento del levante, refer to winds that originate in the

eastern Mediterranean (i.e. Syria, Lebanon, Israel,

Palestine, Jordan, and Iraq) and cross the Sahara Desert. As a result, these winds are composed of hot and dry air,

making them ideal to facilitate the evaporation process.

By employing parabolic trough collectors that heat a

working fluid which passes through a heat exchanger,

submerged in the Salinas channels, it is proposed that the

evaporative mass flux can be increased to promote the

production of sea salt. Additionally, it is proposed that the

production period of the Salinas can be expanded as the

added heat promotes evaporation during time periods

where simple solar radiation does not suffice.

This study therefore presents the results obtained from a

computer simulated model of the system. The simulation

incorporates models of parabolic trough collector

operations [2], a shell and tube heat exchanger model using

NTU method [3], and the modified Meyer evaporation flux

equation [4]. The computer model was programmed using

Engineering Equation Solver (EES), a software tool that

iteratively solves transcendental equations and has built-in

thermodynamic and transport property relations for many

different fluids [5].

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2. BACKGROUND

2.1 The Salinas

The salinas are composed of the estero (“water storage

pond”), three different channel types (the lucios, the retenidas, and the vueltas de Periquito), and finally the

tajos (“crystallization ponds”). An aerial view of a

traditional Salinas can be observed in Figure 1.

The salinas have a slight gradient that allows the water to

flow naturally thanks to gravitational pull. The channels

are created to typically be between 6 and 8 m wide and

between 0.5 and 2 m deep. These dimensions allow the

water to have a flow rate between 0.1 and 0.3 kg/s.

The evaporation, which averaged out throughout an entire

summer, is fairly constant compared to previous years. As

a result it is possible to characterize sections of these

channels with the concentration of the sea water.

Concentration indicators exist for a variety of reasons, be them biological occurrences or physical limitations of

precipitates. For example, different types of plankton can

be found at certain salinities [7]. Depending on the

presence of planktons, the channels walls can have a

certain colored tint. Likewise certain salts (other than

NaCl) precipitate at certain concentrations throughout the

salinas [8]. These precipitate also tint the salina’s walls.

2.2 Meteorological Conditions

According to Figure 2, the Köppen-Geiger Climate

Classification [9] categorizes southern Spain as having dry

and hot summers. Not surprisingly, the province of Cádiz,

including the Bay of Cádiz, suffers from moderate

temperature variations; between 16 C on average during

the winter months and 22 C on average throughout the

summer months [10].

Levante, the easterly or northeasterly winds, result from the

Azores anticyclone weather phenomena. When the

anticyclone extends northeastwardly over Spain and

southern France, it creates a low pressure area over the

western Mediterranean, causing the levante to blow [11].

Typically the levante is a dry wind. The dryness of this wind results from its proximity to Northern Africa. This

can be observed in the Köppen-Geiger Climate

Classification Map (Fig. 2). The map also depicts northern

Africa as a hot and arid desert climate, implying it having

dry air. Because of the Azores anticyclone, air from

northern Africa is suctioned by the pressure differential

previously mentioned, driving the levante over the salinas

of Cádiz and the rest of southern Spain.

2.3 Evaporation Process

According to the Oceanographic Sciences Institute of

Andalucía, the evaporation process at the salinas can be

divided into several stages [12]. Stage 1 begins with

pleamar (“high tide”) when the damn to the estero is

Caño Santi

Petri

Tajos 2

Tajos 1

Retenidas 2 Retenidas 1

Vueltas de

Periquito 1

Vueltas de

Periquito 2

Lúcios

Estero 1

Estero 2

Fig. 1: Property outline of salinas San Vicente. The different

sections depicted include: Estero, Lucios, Retenidas,

Vueltas de Periquito, & Tajos. (Adapted from [6])

1]

N

Fig. 2: Köppen-Geiger Climate Classification for

Mediterranean Ocean. Red depicts: hot, arid desert climate; Yellow depicts: Dry, hot summer climate. (Adapted from

[9])

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opened allowing seawater inflow from the estuary. The

water is trapped in the estero until Stage 2 when a

secondary damn is opened allowing the water to drain into

the lucios. The lucios act as a passage way for the water,

channeling it into the retenidas (“retained water”) where

the evaporation process begins having a significant role. The water is then passed to the vueltas de Periquito for

Stage 3, where salt water reaches concentrations slightly

below the crystallization limit. These 3 stages, last for

several days depending on the climatological patterns and

account for the majority of the evaporation process and

preparation of the highly concentrated sea water. Stage 4

finalizes the process when the highly concentrated sea

water is passed through to the tajos. Here the last bit of

evaporation occurs leading to salt crystallization. At this

point a thin layer of salt is created in the tajos. The full

process is then repeated until enough salt has accumulated

permitting the workers to collect the salt for crushing, processing, and distribution.

3. DATA AND METHODS

3.1 Data

For this analysis hourly data was used to model the system

behavior. Data sets used included meteorological data,

solar radiation data and oceanic data. The data obtained

came from measurement stations, modeled meteorological

conditions, and TMY data.

Figure 3 depicts satellite imagery provided by Google

Earth of the analyzed area and the location of the different

data sets.

The Spanish Port Authority, a department of the Ministry

of Public Works (“Ministerio de Fomento”) provided historical oceanographic and meteorological data from

three different stations. Two of the stations, “Boya Costera

de Cádiz (1320)” and “Boya de Golfo de Cádiz (2342)” are

buoys. The other station, “La Estación Meteorológica de la

Bahía de Cádiz (4340)” is land-based. The scale provided

in Figure 3 allows to approximate the distance from the

stations to the studied salinas.

Surface temperature data was compiled between Stations

2342 and 1320 because the measurement systems aboard

these buoys fail, presenting gaps in the data. Priority was

given to the data obtained from station 1320 due to its proximity to the salinas. Because of its proximity, it is

expected to adequately represent the water temperature of

the salinas, which oscillate between 23 and 26 C during the

summer. This was confirmed by comparing data obtained

in Yúfera’s “Estudio Preliminar del Zooplancton de las

Salinas de Cádiz” [7].

Concentration measurements were obtained from station

2342’s because it was the only equipped with salinity

measurement instrumentation.

Salinas San

Vicente

Fig. 3: Satellite imagery depicting the Bay of Cádiz. The tagged sites depict the location of the corresponding

meteorological stations and Salinas San Vicente (Adapted from [6]).

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Average wind velocity measurements were obtained from

station 4340. This station was selected to provide the wind

data because it is located on a land-based pier. This is

convenient because this site’s wind shear behavior

approximates the wind shear experienced by the salinas

due to surface roughness. The data provided by this station was in 10-minute intervals. As a result, the data was

averaged out throughout each hour to match the hourly

format.

Atmospheric pressure data was provided by the

Meteorological State Agency (“Agencia Estatal de

Meteorología”). The data was also measured from Station

4340. The data provided by this station was presented as a

daily average; as a result the data was distributed over the

24 hours to match the hourly format provided for the other

meteorological conditions.

The Council of Innovation, Sciences and Businesses

(“Consejería de Innovación, Ciencia y Empresa”) of the

Energy Agency of Andalucía (“Agencia Andaluza de la

Energía”) provided TMY data for direct normal solar

radiation. This data corresponds to the area surrounding the

Doña Blanca Castle Station, located at El Puerto de Santa

María. Doña Blanca Castle is approximately 16.8 km from

the studied salinas.

Ambient temperature was provided averaged over 4 hour

intervals by the Weather Research and Forecasting (WRF) model with 9-km resolution. Measured historical data

could not be used because the measurement

instrumentation equipped on the stations failed during the

time period studied for this location. Because the WRF 9-

km model only provides 4-hour intervals, the ambient

temperature was distributed over four hours to match the

hourly format provided for the other meteorological

conditions. There is approximately 10.4 km from the

detailed spot to the studied salinas.

Dew point measurements were obtained from the Global

Summary of the Day provided by the National Oceanic and Atmospheric Administration (NOAA) National Climate

Data Center (NCDC) Geographic Information System

(GIS) Map Services system. The data presented was

measured by the Weather-Bureau-Army-Navy (WBAN)

system for location identifier 13025. This identifier

corresponds to Naval Station Rota, Spain. The data

provided by this station was presented as a daily average;

as a result the data was distributed over 24 hours to match

the hourly data provided for the other variables. Dew point

was used with ambient temperature to calculate relative

humidity. Relative humidity was used with atmospheric pressure and ambient temperature to calculate partial

pressure of water vapor.

Because much of this data set had data holes, only a small

period of time had complete data. The obtained data,

therefore, only allowed analyzing the meteorological

conditions between July 21st, 2004 and September 30,

2004.

3.2 Method

3.2.1 Proposed System

The proposed system is depicted in Figure 4.

Parabolic trough collectors will be installed along the

jetties that shape the channels. These PTC’s should be installed to minimize obstruction from the wind. They will

act as the heat source that feeds the heat exchanger fluid to

the heat exchanger. The proposed heat exchanger will be

comprised of a collection of pipes that facilitate heat

transfer from the working fluid to the salt water. These

pipes would be slightly submerged under the surface of the

water since the temperature of pond solution layer dictates

the evaporation rate of water as presented by Manganaro &

Schwartz [4].

Parabolic Trough Collectors

According to Kalogirou, parabolic trough collectors are a

type of concentrated, sun-tracking collector that can

achieve high temperatures with high efficiency. They are

light structures that provide temperatures in the hundreds

of degrees Celsius at low costs [2]. The collectors are made

up of metal sheets that are bent into a parabolic shape as to

reflect solar radiation to a black metal tube placed along

the focal line [2]. This structure concentrates the solar

radiation from the Sun, heating up the working fluid

running through the black tube.

A horizontal north-south facing collector can collect

slightly more solar radiation than an east-west oriented

collector and also, the north-south collector collects more

Fig. 4: Artistic rendition provided by the author to facilitate

understanding of the proposed system.

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solar energy in the summer months than in the winter [2].

This is critical for the presented project.

A glass cover can be placed around the black tube with a

vacuum seal to reduce convective heat loss, therefore

reducing the overall heat loss coefficient [2]. One disadvantage from this is that the glass provides

transmittance losses [2].

The heat transfer behavior of the parabolic trough collector

was based off of equations obtained from Kalogirou,

S.A.’s Thermal Analysis of Parabolic Trough Collectors

[2].

Equation 1 [2], which describes the useful heat gain, Qu, by

the working fluid,

[ ( )] (1)

In this equation, FR is the collector heat removal factor, Aa,

is aperture area, S is the absorbed solar radiation per unit

area, Ar is the receiver area, UL is the collector overall heat

loss coefficient, Ti is the initial temperature, and Ta is the

ambient temperature.

Equation 2 describes the outlet temperature [2], To, after

the fluid has absorbed all the useful heat gain:

(2)

Ti is the inlet temperature, Qu is the useful heat gain from

Equation 1, is the mass flow rate, and Cp is the heat

capacity.

Heat Exchangers

A shell and tube heat exchanger was used to approximate

the heat transfer behavior from the working fluid to the sea

water in the salinas’ channel. Figure 5 depicts what a heat

exchanger of this type looks like. This approximation was

used since the sea water flows through a confined space

over tubes that transport the working fluid.

For the analysis of this heat exchanger the effective Number of Transfer Units (NTU) method was used, rather

than the log-mean temperature difference method. This is

because the NTU method is better at predicting the outlet

temperatures of the hot and cold fluid streams in a

specified heat exchanger [3].

This model can be used because heat exchangers usually

operate for long periods of time with no change in the

operating conditions [3]. For such steady-flow conditions

to be achieved it is important that the mass flow rate of

both fluids and their temperatures remain constant.

The specific heat of a fluid will change with temperature

but over a specified temperature range it can be treated as a

constant at some average value with little loss in accuracy.

Also, the heat exchanger is assumed to be perfectly

insulated, so that there is no heat loss to the surrounding

medium, and any heat transfer occurs only between the two

fluids [3].

Some of these approximations are not true in the proposed

system as the inlet temperature of water can change due to

the atmospheric conditions and because the proposed heat exchanger is not completely enclosed. But due to the

complexity of the proposed system, the approximation of

the heat exchanger will suffice for this study.

Equations 3 and 4 are relevant in modeling this system’s

behavior as they provide the outlet temperature for both the

sea water and the working fluid, respectively [3].

(3)

(4)

Tc,out is the outlet temperature of the seawater, Tc,in is the

inlet temperature of the sea water, Q is the actual rate of

heat transfer between fluids (same for both fluids), Cc is

the heat capacity rate of the working fluid, Th,out is the

outlet temperature of the working fluid, Th,in is the inlet

temperature of the working fluid, and Ch is the heat

capacity rate of the seawater.

It is also important to note that the NTU method does not

consider material properties. This is significant as in reality

the material used would be limited to those that are

corrosion resistant due to the corrosive behavior of

seawater. In such a case, it is proposed for future studies

Fig. 5: 1-shell-pass and n-tube-passes Heat Exchanger.

(Adapted from [3])

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consider analyzing stainless steel 316. This metal is highly

corrosion resistant and used in many maritime applications.

Also, it is important to address the working fluid used to

transfer heat from the parabolic trough collectors to the

salinas. It is proposed that water be the fluid of choice as it would not cause any internal corrosion of the system.

Likewise, in the case of system failure and leakage, water

would not contaminate the salinas. This is an important

consideration because the salinas must meet certain criteria

and regulations imposed on the food processing industry.

Evaporation Flux

This study uses the evaporation flux equation proposed by

Manganaro and Schwartz’s [4]. In brief, this section will

summarize the theory presented in this source.

Figure 6 presents a control volume for a solar salt pond

and also depicts the material and energy flows used to

model the evaporation process.

From this model and using a modified version of the

Meyer equation, Manganaro and Schwartz were able to

devise the Evaporation Flux equation (Equation 5), re, which governs a solar evaporative process for a pond in

terms of lb/hr-ft2.

( )( ) (5)

The fundamental variables driving the evaporation flux

equation include: the velocity of the wind, V3, measured in

mph; the partial pressure of water vapor in air, p3,

measured in mmHg; the geometric constant, C, which

usually is 0.5 for shallow pans or 0.36 for ponds or lakes; and the calculated vapor pressure of the water over the

pond solution, p1, provided in mmHg. Equation 6 describes

how all these variables come together,

( ) (6)

As shown, the vapor pressure of the water over the pond

solution depends on: the salt concentration in solution

bulk, c1, measured in terms of weight-fraction, and the

vapor pressure of pure water, pw0, measured in mmHg. The

vapor pressure of pure water may be calculated given the

temperature of pond solution layer, T1, measured in oC. The vapor pressure of pure water can be calculated with

Equation 7.

[

( )

) (7)

4. RESULTS & DISCUSSION

The system analyzed comprised of a 20 m parabolic trough

collector with a 3.5 m wide aperture that had a reflectance

coefficient of 0.86. The reflected radiation’s focal point

focuses on a receiver surrounded by a glass cover, forming

a vacuum between the receiver and the cover. The glass

cover has a diameter of 0.09 m and emissivity of 0.87. The

receiver, modeled as stainless steel with blackened nickel

surface treatment, has a thermal conductivity of 15 W/m-

K, and an outer diameter of 0.05 m with a wall thickness of 0.0005 m. The receiver absorptivity is 0.97, the

transmissivity is 0.80, and the emissivity is 0.92. The heat

transfer coefficient inside the receiver is assumed to be 330

W/m2-K.

Water was used as the working fluid passing through the

parabolic trough collector and the heat exchanger because

it is non-toxic in the case a spill is to occur. Specific heat

of water was modeled to have 4250 J/kg-K and a mass

flow rate of 0.32 kg/s.

The heat exchanger was modeled as having 1-shell and 80-

tube passes, each tube having a length of 5 m. It was

estimated that the overall heat transfer coefficient was 310

W/m2-K.

The cold fluid entering the heat exchanger was the sea

water at 20 C. Seawater at this temperature correlates to a

specific heat of 3,940 J/kg-K.

For the evaporation rate, the constant C is usually 0.5 for

shallow pans or puddles and about 0.36 for a pond or lake.

For the salinas, C was estimated to be 0.36 [4].

Inputting the data above with the previously described

meteorological data into the EES model produced the plot

depicted in Figure 7 which depicts the evaporation rate of

the salt water in the Salinas. The blue line depicts the

evaporation rate resulting from the enhanced PTC and heat

exchanger system. The red line depicts the evaporation rate

resulting from traditional methods.

Fig. 6: Solar salt and pond model. (Adapted from [4])

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The peaks depicted in the graph correspond to the day time

hours, while the lower values correspond to night hours.

These peaks occur because evaporation increases during

the day time when the wind and solar radiation combine to

promote this effect. At night, with 0 W/m2direct normal

solar radiation, the parabolic trough collectors do produce the heat needed to accelerate the evaporation process.

As it can be observed from Figure 7, the evaporation rate

of the sea water is drastically increased with the enhanced

system compared to the evaporation rate through the

natural process. It is estimated that the enhanced system

evaporates water six to ten times faster than the natural

process.

5. CONCLUSION

According to the EES model, the implementation of the

parabolic trough collectors and heat exchangers to the

traditional salters could drastically increase the sea salt

production efficiency of the salinas. Therefore, by

increasing the production rates, it is possible that the

salinas can regain competitive market position.

It is recommended that further investigation to this topic

include payback analysis. Such analysis is required to

examine if investing in parabolic trough collectors and heat

exchangers to increase the evaporation rate of water is

feasible.

At the same time, it is important to note that this model is

based on theoretical calculations and estimated values,

therefore limitations to its accuracy exist. As a result,

experimental research is necessary in order to obtain

realistic measurements of the system. This will allow

updating the computational model to depict more

accurately the evaporation of the salinas.

Experimental research should address heat exchanger

options, as the shell and tube heat exchanger is not the

most adequate model. Also, it is advised to study the

behavior of the working fluid as phase changes can potentially effect the heat transfer dynamics. The data

obtained from this research would allow to furthering the

accuracy of this assessment.

0

5

10

15

20

25

30

35

40

4850 5050 5250 5450 5650 5850 6050 6250 6450

Evap

ora

tio

n F

lux,

kg/

m^2

-hr

Hourly Date in Year, hr

Salinas Evaporation Rates Enhanced

Natural

Sept 30, 2004

July 21, 2004

Fig. 7: Results graphing the evaporation rates of the enhanced model versus the natural system.

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6. REFERENCES

(1) Villalobos, C.A. and Garcia Prieto, F.J. and Menan-

teau, L. (2003) Las salinas de la bahia de Cadiz

durante la Antigüedad: Vision Geoarqueolgica de un Problema Histrico. SPAL 12 (2003): 317 - 332.

Website:

http://dialnet.unirioja.es/servlet/fichero_articulo?codig

o=1083665&orden=0

(2) Kalogirou, S. A. (2009). Solar Energy Engineering:

Processes and Systems. (pp. 121-212 & 199 - 204).

London: Academic Press.

(3) Cengel, Y. A. (2003). Heat Transfer: A Practical

Approach. 2nd Ed. McGraw Hill.

(4) Manganaro, J. L., & Schwartz, J. C. (1985).

Simulation of an evaporative solar salt pond. In Ind.

Eng Chem. Process Des. Dev. (24 ed. pp. 1245-1251). American Chemical Society.

(5) Klein, S.A. and Alvarado, F.L. (2011). Engineering

Equation Solver, F-Chart Software, Academic Version

(insert the number from ‘Help’ -> ‘About EES’ in

EES).

(6) Map of the Bay of Cádiz, Cádiz, Spain. Google Earth.

(36.446N, 6.164E)

(7) Yúfera, M., Lubián, L.M., and Pascual, E. (1984).

Estudio preliminar del zooplancton de las salinas de

Cadiz. Limnetica 1: 62-69. Asociación Española de

Limnologia, Madrid, Spain. (8) Ruiz Coto, Manuel. Salina de San Vicente Video: II.

Recorded Interview, Youtube.

http://www.youtube.com/watch?v=4vFdnNk1jg0&feat

ure=player_embedded

(9) Peel, M. C. and Finlayson, B. L. and McMahon, T. A.

(2007). "Updated world map of the Köppen-Geiger

climate classification". Hydrol. Earth Syst. Sci. 11:

1633-1644. ISSN 1027-5606

(10) Boya de Golfo de Cadiz. Datos Historicos: Salinidad.

Informes Anuales de todos los sensores de la estación.

Hidrografía (2009). “Análisis de los datos de salinidad

y temperatura”. Puertos del Estado de España. http://www.puertos.es/oceanografia_y_meteorologia/r

edes_de_medida/index.html

(11) Reiter, E. R. in Naval Postgraduate School,

(1975).Handbook for forecasters the mediterranean:

Weather phenomena of the mediterranean basin.

Retrieved from Environmental Prediction Research

Facility website:

http://www.nrlmry.navy.mil/pubs/forecaster_handboo

ks/Med_1/

(12) Instituto de Ciencias Marinas de Andaluca (ICMAN).

(2011). "Salinas Tradicionales." Consejo Superior de Investigaciones Cientificas (CSIC) Spanish National

Research Council. Web. 17 Apr. 2011. Website:

http://www.icman.csic.es/salinasdelabahia.tradicional.

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