experimental and theoretical investigation to generate...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:06 11

I J E N S IJENS © December 2018 IJENS -IJMME-4747-061821

Experimental and Theoretical Investigation to

Generate Steam by Parabolic Trough Solar

Collector with Using Different Heat Transfer Fluids Mohammed Hasan Abbood

1 and Mohammed Mohsen Mohammed

2

1 PhD, Assistant professor, Mechanical Engineering Dep., College of Engineering, University of Kerbala.

2 M.Sc. student, Mechanical Engineering Dep., College of Engineering, University of Kerbala.

Abstract-- This paper describes the mathematical model and

experimental tests for parabolic trough solar collector (PTSC).

The model is based on detailed energy balances, and it has been

applied to evaluate collector thermal performances with

different heat transfer fluids. The influence of fluids

temperature has been studied from the point of view of heat

gain and thermal efficiency. The working fluids which have

been selected for this study are hydraulic oil, ethylene glycol

based water, and water. An experimental investigation for

testing the performance of parabolic trough solar collector

(PTSC) is carried out to generate steam at moderate

temperature. The tests have been carried out in KERBALA

climatic conditions (32.34º N, 44.03º E) during selective days.

The results show that the best performance was in case of using

hydraulic oil as heat transfer fluid. The maximum

enhancement in thermal efficiency was 31.7% between

hydraulic oil and water. While the enhancement in the thermal

efficiency reaches about 20.4% between ethylene glycol and

water.

Index Term-- Solar collector, Concentrated solar collector,

Parabolic trough collector, Receiver tube, Thermal efficiency.

NOMENCLATURE

Aa Collector Aperture area K(Ɵ) Incident Angle Modifier

Ag Area of Glass Cover L Collector length

Ar Area of Receiver Nu Nusselt Number

Cp Specific Heat Pr Prandtl Number

Dgc Glass Cover Diameter Qu Useful Heat Gain by the collector

Dr Receiver Diameter Re Reynolds Number

FR Heat Removal Factor S Absorbed Solar Radiation Per Unit Area

hrad,gc-a Radiation Heat Transfer Coefficient between

Glass and Ambient Air Ta Ambient Temperature

hrad,rec-gc Radiation Heat Transfer Coefficient between

Receiver and Glass Cover Tg Temperature of the Glass Cover

hconv,rec-gc Convection Heat Transfer Coefficient between

Receiver and Glass Cover Tf,i Inlet Fluid Temperature to the Collector

hw Wind Heat Transfer Coefficient Tf,o Outlet Fluid Temperature from the

Collector

Ib Beam Radiation Tw Water temperature inside the heat

exchanger

K Thermal Conductivity UL Heat Loss Coefficient

1. INTRODUCTION

In recent years, competition and development among

countries to produce energy cleanly away from traditional

ways of producing energy with very large residues on the

environment. Many countries now rely more on renewable

energy and are working to transform them from one form to

another and develop them. One of the most important

renewable energy sources that can be depended on the solar

energy. Most the application of steam generation for

industrial use or electricity production by solar energy, used

the method of concentrator solar radiation. Nowadays,

parabolic trough solar technology is the most used solar

application. The following researches will show the

summary for previous theoretical and practical work

regarding parabolic trough system.

The first practical experience with Parabolic trough

solar collectors (PTSC) goes back to 1870, when a

successful engineer, Ericsson, a Swedish immigrant to the

United States, designed and built a 3.5m2 aperture collector

which drove a small 373 W engine. Steam was produced

directly inside the solar collector (today called Direct Steam

Generation or DSG) [1]. Following this more researches are

try to enhancement the efficiency of PTSC. A publication

then by Lüpfert et al. [2] studied experimentally thermal

properties of receivers of a parabolic trough. In this study

different methods to calculate the thermal loss from a

receiver tube was presented based on their operating

temperature. It was observed that the solar parabolic trough

plants which are having a temperature range of about 390°C

were having an energy loss of about 300 W/m of receiver

length. Qu et al. [3] developed a mathematical model of



parabolic trough collector used for solar cooling and heating

by using energy balance correlations between the absorber

tube, glass tube, and surroundings. The results of the

comparison between the mathematical model and the

experimental data indicate some differences, including high

measured glass temperature and low measured efficiencies.

These differences are attributed to heat loss at the supports

and the connectors and the low assumption of the

absorptivity. Ouagued et al. [4] built a numerical model of a

parabolic trough collector under Algerian climate. In this

model, the receiver is divided into several segments, and

heat transfer balance equations which rely on the collector

type, optical properties, heat transfer fluid (HTF), and

ambient conditions are applied for each segment. This work

led to the prediction of temperatures, heat loss, and heat gain

of the parabolic trough. Raj et al. [5] performed numerical

and experimental analysis on the performance of the

absorber tube of a parabolic trough collector system with

and without insertion. Water was used as the heat transfer

fluid and three different mass flow rates had been used

which were 33Kg/hr, 63 Kg/hr and 85 Kg/hr. It was seen

that presence of inserts gave a higher rise in the outlet

temperature when compared with the one without insertion

which was due to the fact the area of heat transfer was

increased with the insertion. It was also observed that the

thermal stresses on the tube with insertion were less than

that without insertion. A study then done at 2014 by Filho et

al. [6] to describes the methodology and the results of an

experimental and numerical investigation of the thermal

losses of a small scale parabolic trough collector, The

numerical simulation when compared to the experimental

results for the losses shows the degradation of the vacuum in

the annular region of the absorber tube also the collector

was tested with solar radiation and the efficiency obtained

varied from 0.3 to 0.55.

The present work aims to develop a mathematical

model and experimental tests for parabolic trough solar

collector to generate steam by using different types of heat

transfer fluid (HTF) and with flow rate of 3 liters per minute

(LPM).

2. PARABOLIC THROUGH SOLAR COLLECTOR

Parabolic trough solar collectors (PTSC) are

considered as one of the most mature, successful, and

proven solar technologies for electricity generation and

steam requirements. The PTSC are typically operated at

temperatures range 50-400 ºC. The PTSC system consists of

support structure, parabolic trough, and receiver tube. The

parabolic trough shaped mirror concentrate sun rays onto

receiver tube which is placed in the focal line of the

parabola trough as shown in figure (1). The solar parabolic

trough is made up of several reflectors. The receiver tube is

typically composed of glass cover and an absorber tube with

a vacuum between these two elements, to reduce heat losses

by convection. The receiver of PTSC usually paint with

selective coating in order to maximize solar radiation

absorption. The absorbed solar radiation is conveyed to the

heat transfer fluid (HTF) that flows inside the absorber tube.

The HTF is provided heat to the thermal system directly by

using the fluid or indirectly by transferring heat using a heat

exchanger.

3. Heat transfer fluid (HTF)

Parabolic trough solar collectors PTSC used heat

transfer fluid HTF that flows through the receiver tube,

which is collecting and transporting solar thermal energy to

storage tanks or heat exchanger. The HTF leaves the

parabolic solar collector is utilized to produce steam that

used either in industrial processes or to generate electricity.

Some criteria must be taken in considered when choosing a

HTF such as thermal capacity, the coefficient of expansion,

viscosity, boiling point and freezing point. The most

commonly HTFs used in the applications of PTSC are air,

water, glycol and hydrocarbons oil.

Fig.1. The PTSC system

4. MODEL ANALYSIS

A theoretical model of PTSC has been presented in

detail. The model is mainly consisting of three parts. The

first part is estimating the solar radiation that reaches the

collector. The second part is optical analysis. The purpose of

this section is to determine the optical efficiency of the

PTSC which is the ratio of the amount of energy arriving to

the absorber tube to that hits the reflector. The last part is

thermal analysis, and the aim of this part is to predict the

thermal efficiency of PTSC. The proposed model is

implemented is MATLAB software, and it is validated with



experimental data. So the equations of this model can be

describe here as:

Optical efficiency

The optical efficiency is defined as the ratio of energy

reaching the absorber tube to that received by the reflector

aperture [7] :

(1)

The actual amount of absorbed radiation on the receiver is

calculated by [8]:

( ) ( ) (2)

Thermal model of PTSC

The thermal model was based on a one-dimensional steady-

state thermal-resistance heat transfer model. This model can

predict the behavior of any parabolic collector design under

any ambient condition and with a variety of work

parameters [9]. The heat transfer model considers radiation

and convection heat transfer modes from the solar radiation

to heat transfer fluid (HTF) as shown in figure (2).

Fig. 2. Thermal resistance model

Heat loss coefficient (UL) can be estimate based on receiver area:

[

( )

]

(3)

For the space between the glass cover and receiver is evacuated, and convection losses are negligible, UL is calculating from the

following equation:

[

( )

]

(4)

The convection heat transfer coefficient between the glass cover and ambient due to wind (hw) [7]:

(5)

For the convection loss coefficient, the Nusselt number (Nu) and the Reynold’s Number (Re) can be used as follow [7]:

( ) (6)

( ) (7)

(8)

The radiation heat transfer coefficient between glass cover and sky temperature is:

( ) (

) (9)



Where sky temperature is calculating from the following equation [10]

(10)

The radiation heat transfer coefficient between glass cover and receiver is:

( ) (

)

(

)

(11)

Heat Transfer to Working Fluid

The heat transfer from the receiver tube to the fluid (HTF) must be characterized by turbulent or laminar flow conditions

accordingly, the evaluates the Reynolds number of the fluid [11].

(12)

Nusselt number of the fluid :

(13)

( ) ( ) (14)

The heat transfer coefficient to the fluid, is evaluated:

(15)

Overall Heat Transfer Coefficient and heat removal Factor

The overall heat transfer coefficient, Uo, needs to be estimated, this should include the tube walls because the heat flux in a

concentrating collector is high. Based on the outside tube diameter, this is given by [11]

[

(

)

]

(16)

The collector efficiency factor, F', is given as:

(

)

(17)

The collector heat-removal factor FR, it is one of important design parameter where it is a measure of the thermal

resistance encountered by the absorbed solar radiation that reaching the collector. Its value depends on flow rate of working

fluid inside the receiver, FR is found by this equation [12]:

[ (

)] (18)

Thermal Efficiency of parabolic trough collector

The thermal efficiency of a PTSC can be defined as the ratio of heat gained by the collector, Qu, to the total incident

radiation that is incident on the aperture of the collector

(19)

Where the useful heat gained, Qu, is a function of the outlet and inlet temperature of the heat transfer fluid in the receiver tube as

shown below:

( ) (20)

The useful heat collected by the receiver can also be expressed in terms of optical efficiency, heat loss coefficient, heat removal

factor, and inlet receiver fluid temperature:

[

( )] (21)

Thermal efficiency can be found using the following equation [11]:



( )

(22)

5. Numerical solution for a mathematical model

The mathematical model is applied to study the

performance of a parabolic trough solar collector (PTSC).

This model consists of three parts. The first one is a solar

radiation model, the amount of beam radiation incident upon

collector is calculating using relationships and equations

between the Earth and the sun. The second one is the optical

model. In this part, has the ability to estimate the optical

efficiency. The last one, is the thermal model, which

estimate the amount of energy collected by the working

fluid, heat loss and thermal efficiency. This model is

implemented by MATLAB software. There are two types of

inputs: input variables and input parameters. The input

variables are changeable through the daytime such as date,

time, ambient temperature and wind speed. These input

variables must be entered into the code in order to run the

simulation. The input parameters have fixed values and do

not change through day time such as geometrical properties,

latitude and longitude of the PTSC location, outer and inner

diameter of the glass envelope and the receiver, aperture

area of the trough, and optical properties.

Fig. 3. Flow chart of MATLAB

For the solar model, inputs are time, and day date. All

the equation and relationship of sun-earth are considered to

produce angle of incidence and beam radiation. For optical

model, which the inputs are emissivity, absorptivity,

transmisivity, and reflectivity of the tube and parabolic

trough. Also, Incident angle modifier and End loss factor are

solved, which used to estimate the optical efficiency of

PTSC. For thermal model in which consist of heat transfer

equations by convection and radiation.

To find all temperatures and solve these equations, an

iterative process was suggested by Duffie [13]. Procedure

that used to solve this equation is by initially guessing a

value (closer to ambient temperature) for the outer glass

temperature, then solve the heat transfer equation through

heat collection element and evaluate the inner glass

temperature. The outer glass temperature (initial guess)

needs to be checked by performing an energy balance on the

glass cover to evaluated new value of outer glass

temperature. By comparing the two values of outer glass

temperature. If the comparison is not equal, a new guessing

value will apply as shown in flow chat, figure (3). after all

temperatures required are evaluated, heat losses, heat gain

and thermal efficiency are calculated.

6. Experimental rig and test procedure

A PTSC is a concentrating collector, which the

reflector surface has a parabolic form as shown in figure (4).

Solar radiation falling on trough then concentrated on the

focal line, where a receiver tube was placed. The parabolic

collector consists of three troughs, which are arranged in

series so that the heat transfer fluid (HTF) will gain heat

gradually as it flows through the tubes one by one. By

circulating the HTF, thermal energy in the HTF will transfer

to water by using a coil tank heat exchanger.

The experimental rig is designed, constructed and

tested to investigate its ability for steam generation at

moderate temperature. The PTSC system consist of a

mechanical structure (support frame), absorber, reflecting

parts, evacuated glass tube, heat exchanger, centrifugal

pump, auto tracking system and some other accessories. The

PTSC specifications are given in table 1. The glass receiver

tube composed of two coaxial borosilicate glass tubes with

one open end and another sealed.



Fig. 4. The PTSC system

Table I

Specifications of the PTSC

ITEM Value/Type

Rim angle (θr) 77.72

Focal distance (f) 233.1 mm

Glass cover external diameter (Dg) 58 mm

Receiver external diameter (Dr) 47 mm

Collector aperture width (W) 1040 mm

Concentration ratio (C) 46.8

The effective aperture width (W) 750 mm

Collector length (L) 1800 mm

Parabolic curvature 1220 mm

Total Collector aperture area 3.73 m2

Reflective material Aluminum composite panels

Tracking system One axis auto tracking system

Electrical motor Electrical motor with moving arm

The experimental setup used for testing the PTSC

system shown schematically in Fig. 5. It consists of the

following (1) support structure, (2) three parabolic troughs

(3) evacuated glass tube (4) coil-tank heat exchanger, and

(5) auto tracking system.

First, it is very important to clean the reflector from

any dirt or dust. Then, pump and tracking motor wires are

connected to electricity to running the test. Heat exchanger

tank is filled with 30 liters of water (half volume is filled

with water only). In the current experiment work, the

working fluid is circulated in a closed cycle. The collecting

tank is filled with working fluid from the main supply. After

this tank had been filled, the outlet from the tank connecting

to the pump, then the working fluid is pumped to the inlet of

troughs and the trough outlet to the coil inlet then exit from

the coil is connecting again to collecting tank. After that, the

tracking system of the troughs is turned on and moves the

troughs until the sun is directly over the troughs. The flow

meter keeps the flow rate of working fluid at 3 LPM, which

is placed before the troughs inlet and after the pump. The

fluid temperatures in their locations (as mentioned in

schematic diagram), ambient temperature, wind speed, the

pressure of steam and solar radiation are continuously

recorded during the test. After starting the test, working

fluid collects heat and its temperature rises, it goes to heat

exchanger coil to transferring heat energy to water. This

cycle repeats continuously throughout the test period until

steam production is achieved. In the current work, three type

of working fluid hydraulic oil and ethylene glycol based

water and water have been used.



Fig. 5. Schematic Diagram of Experimental Setup.

7. Results and discussion

This study presents the experimental and theoretical

investigation to generate steam by using a parabolic trough

solar collector PTSC system. Thermal performance for

PTSC is determined theoretically in MATLAB (R2014a)

software by evaluating the parameters that effected on the

system such as solar radiation, wind speed, day number, and

temperatures of HTF. Also, the thermal performance of

PTSC was calculated from measured data experimentally.

The measured data was recorded with different heat transfer

fluid hydraulic oil, ethylene glycol, and water. The tests

were conducted at the college of engineering/ University of

Kerbala (44.03° Longitude and 32.34° Latitude), through the

clear sky days for the months of July and August from 9:00

A.M until 2:30 P.M,

8. Outlet temperature

In this cases, the data measured experimentally are

recorded for three days under clear sky similar weather

(25th of July, 5th

and 9th

of August, 2018). all the necessary

data have been measured to study the performance of the

PTSC with hydraulic oil, ethylene glycol, and water as HTF.

Figure (7) shows the outlet temperature of working fluid

from the collector for different HTFs, which is increase

through the test day with time passing for all type of the

HTF until it reaches to the end of the experiments. This can

be referring to the actuality that the incident solar radiation

which falls on the collector directly due to auto tracking

system of the collector continuously and directly toward the

solar radiation. The maximum difference between hydraulic

oil and water was 21%, while the difference between

ethylene glycol and water was 7%. Figure (8) shows the

temperature difference between the outlet temperature and

inlet temperature through the solar collector for each

working fluid. This is referring to hydraulic oil is better than

ethylene glycol and water, which is heated up faster and its

temperature rises more. this is due to the low heat capacity

of hydraulic oil as compared with the two other fluids.

8.1 Heat gain through the collector

The useful heat gained was calculated from the inlet

and outlet temperatures through the collector, specific heat,

and flow rate of HTF. Figures (9) and (10) show the

experimental and theoretical heat gained from the collector

with time at 3 LPM flow rate respectively. It was noticed

from experimental results at figure (9), the maximum heat

gained enhancement between the hydraulic oil and water

was 29%, while the maximum enhancement between the

ethylene glycol and water was 16.5%. The useful heat gain

for the hydraulic oil is better than the other two HTF, and

the heat gain for ethylene glycol is better than the water and

lower than the hydraulic oil. This is because of the

difference in the thermal properties of each type of fluid

used and their ability to raise its temperature. The

comparison between the experimental and theoretical results

show that the maximum difference is 10.4%, 16%, and

14.9% for hydraulic oil, ethylene glycol based water, and

water respectively. This is due to several factors such as the

assumptions that assumed during the implementing of the

model, applying different equations, correlations, and Earth-

sun relationship in the model, the effect of dust and clouds

on the solar radiation that measured experimentally is

significant when compared with theoretical solar radiation,

and also thermal and optical losses that calculated in this

model can be lower than that the actual losses that have been

significantly influenced by the results of experimental heat

gain.

8.2 Heat gained by the heat exchanger tank

The heat gained by the water in the heat exchanger

tank is computed every 15 minutes through the tests with

different HTF (hydraulic oil, ethylene glycol, and water).

Figures (11), (12), and (13) show the comparison between

the three types of HTF. It is found that the temperature of

water in the tank of heat exchanger increase with time

passes. As a result of the solar radiation increase with time

passes. It is increased from 48.2oC to 95.4

oC, 47.9

oC to

92.1oC, and 47.8

oC to 90.6

oC for hydraulic oil, ethylene

glycol, and water, respectively.

On the other hand, the heat gained by the water starts

to increase until reach the maximum value 0.797 KW, 0.711

KW, and 0.58 KW, for hydraulic oil, ethylene glycol, and

water around noontime, respectively. Then, start to decrease

slowly due to decreasing in the solar radiation. This is due to

the proportional relation between the heat gain through the

collector and the heat gained by the water in the tank of the



heat exchanger, and their relation with a variation of solar

radiation during the test period.

8.3 Thermal efficiency

The collector thermal instantaneous efficiency is

determined from useful heat gain, solar radiation, and

aperture area, to analyze the performance of the PTSC

system. Figure (14) shows the comparison between the

experimental thermal efficiency for different HTF hydraulic

oil, ethylene glycol based water, and water. The maximum

difference in thermal efficiency was 31.7% between

hydraulic oil and water. While the difference between

thermal efficiency reaches about 20.4% between ethylene

glycol and water. It is clear that hydraulic oil gives better

thermal efficiency than other HTF, and thermal efficiency of

ethylene glycol is higher than water but lower than hydraulic

oil with the same flow rate. This is due to the different heat

capacity of HTF. Figure (14) and (15) show the

experimental and theoretical thermal efficiency. The

comparison between the experimental and theoretical results

show that the maximum difference is 9.7%, 15.1%, and

21.1% for hydraulic oil, ethylene glycol based water, and

water respectively. This is due to several factors such as

dust, clouds, wind speed, the transmissivity of the glass

cover, and absorptivity.

Fig. 7. outlet temperatures for different HTF with time at 3 LPM flow rate

Fig. 8. temperatures difference between inlet and outlet for different HTF

with time at 3 LPM flow rate

45

55

65

75

85

95

105

115

125

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Tem

per

ature

(oC

)

Time (hr.)

Hydraulic Oil Ethylene Glycol Water

0

2

4

6

8

10

12

14

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Tem

per

ature

dif

fere

nce

(oC

)

Time (hr.)

Hydraulic Oil Ethylene Glycol Water



Fig. 9. experimental heat gain for different HTF with time at flow rate 3 LPM

Fig. 10. theoretical heat gain for different HTF with time at flow rate 3 LPM

0

0.2

0.4

0.6

0.8

1

1.2

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Usf

ul

hea

t gai

n (

Kw

)

Time (hr.)

water

ethylene glycol

hydraulic oil

0

0.2

0.4

0.6

0.8

1

1.2

1.4

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Usf

ul

hea

t gai

n (

Kw

)

Time (hr.)

water

ethylene glycol

hydraulic oil



Fig. 11. experimental heat gained by water and steam temperature with

time for hydraulic oil as working fluid


time for ethylene glycol as working fluid

40

50

60

70

80

90

100

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Ste

am t

emper

ature

(oC

)

Usf

ul

hea

t gai

n (

Kw

)

Time (hr.)

Useful Heat Gained by

water

40

50

60

70

80

90

100

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

9:00 AM 10:00

AM

11:00

AM

12:00 PM 1:00 PM 2:00 PM

Ste

am t

emper

ature

(oC

)

Usf

ul

hea

t gai

n (

Kw

)

Time (hr.)

Useful Heat Gain by

water




time for water as working fluid

Fig. 14. experimental thermal efficiency at different HTF with time

40

50

60

70

80

90

100

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

9:00 AM 10:00

AM

11:00

AM

12:00 PM 1:00 PM 2:00 PM

Ste

am t

emper

ature

(oC

)

Usf

ul

hea

t gai

n (

Kw

)

Time (hr.)

Useful Heat Gain by water

Steam Temperature

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM

Eff

icie

ncy

Time (hr.)

water

ethylene glycol

hydraulic oil



Fig. 15. theoretical thermal efficiency at different HTF with time

9. Conclusions

A mathematical model has been implemented for

studied the thermal performance of the PTSC so as to

generate steam. This model takes into consideration the heat

transfer equations and correlations and heat transfer

mechanism that including radiation and convection. An

experimental investigation has been carried out for

analyzing the thermal performance of PTSC experimentally

so as to generate steam at medium temperature. In this

study, the experimental tests carried out with different heat

transfer fluids HTFs. The performance of the PTSC system

depends on the measured parameters such as ambient

temperature, inlet temperature, outlet temperature, and solar

intensity. A peak experimental efficiencies close to (33.2%,

28.5%, and 22.7%) were obtained for parabolic trough

collectors with hydraulic oil, ethylene glycol based water,

and water respectively.

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experimental and theoretical investigation to generate...

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