performance evaluation of concentered trough solar

Performance Evaluation of Concentered Trough Solar Collectors

Based On Connection Mode

Bushra S. Younis1,∗, Karima E. Amori 2. 1,2 Department of Mechanical Engineering, College of Engineering, University of

Baghdad, Iraq.

1. Introduction

The solar collector is designed and used to absorb thermal energy from solar radiation. The solar energy

is used, in several fields such as a solar furnace, heating swimming pool, industrial process heat supply,

desalination, heating water, and thermal power. Flat plate collector, parabolic trough collector (PTC),

parabolic dish collector (PDC) and solar power towers (SPT) are four types of solar collector field that operate

in the various temperature range. The thermal performance of these solar collectors was studied by several

scholars. Compared Combined Heat And Power Solar (CHAPS) system with a flat plate collector. Found the

system has 58% thermal efficiency that depends on temperature difference, 11% electrical efficiency that

depends on absolute temperature and 69% combined efficiency, showed that when the two-system operating

at low temperature, the CHAPS collector has efficiency lower than a flat collector, but when to increase

operate temperature decreases the difference in their performance [1]. Lower operation and maintenance

(O&M) cost of parabolic trough collector (PTC), when compared with linear Fresnel collector (LFC), is

reported. O&M cost of LFC is in a range from 28% to 79% relative to PTC, while thermal performance for

PTC is better than LFC [2]. The best locations to collect the concentrated solar radiation is studied, by

developing the basic equations to calculate the extra irradiation data monthly. Results show there are eight

locations in the southern and northern of hemisphere, it can select in the collector. Also, shows Parabolic Dish

Collector (PDC) and Solar Power Towers (SPT) have expensive costs relative to Parabolic Trough Collectors

(PTC) and Linear Fresnel Reflector (LFR). And PTC needs more space than PDC and LFR in the CSP

system, while PDC needs less space than the others [3]. Compared to solar collectors with internal finned

absorber systems and a smooth absorber system, to determine the optimal fin dimension with different criteria,

using twelve absorbers have inner fins with different thicknesses (6mm,4mm,2mm) and different lengths

(20mm,15mm,10mm,5mm). Showed that the thermal efficiency first type is improved by 1.27% by using

internally finned absorbers [4]. Plastic absorber with fife differrent values of water flowrate is studied, shows

the maxiumum value of temperature difference is 3.1℃, and maximum thermal efficiency value is 79% [5].

The effect of absorber length on the value of the heat transfer coefficient is studied, using four different

absorber length with different number of turns. Shows the heat transfer coefficient is directly proportional to

the number of turns, due to turbulent flow in certain areas [6]. The Comparing solar collectors tracking, with a

fixed collector, shows higher energy absorbed from solar radiation by 46.46% when used tracking collector

[7]. Comparing rotatable axis tracking. with a fixed collector, showed that the efficiency of a solar collector is

improved by 5% by using the tracking collector [8]. The optical efficiency of the solar collector system is

improved, by using a parabolic trough flux scanner with an instrument, for measuring the density of solar flux

[9]. Also, the optical efficiency of the solar collector system is improved to 54.2%, by improvement in the

structure design of parabolic, and by choice type of absorber materials [10]. The built of the parabolic trough

collector in several countries, and where its use, are studied, during the past period [11]. The effect of

collector dimension on the optical efficiency is studied, by using different sizes of parabolic trough collectors.

Proved that the increase in the optical efficiency of the collector is due to reducing width aperture and increase

rim angle [12]. Comparing numerical results with experimental data, showing how the temperature is

distributed [13]. The effects parameters such as (weather conditions, solar radiation, steam drum volume), on

collector efficiency are studied. Proved the solar collector has better performance at high thermal energy, and

mid-temperature [14]. In the present work, the performance of the connection type (parallel or series) of two

Journal of Xi'an University of Architecture & Technology

Volume XII, Issue II, 2020

Issn No : 1006-7930

Page No: 2071

units of parabolic trough solar collectors is studied. The performance of the collector system can be enhanced

by adopting a better connection. The temperature of oil, absorber, and Glass cover, the pressure drop, and the

mass flow rate for each connection are accomplished under Baghdad-Iraq climate conditions.

Nomenclature

Symbols

A Area(𝑚2)

D Diameter(m)

F Focal length (m)

G Actual amount of absorbed energy(W/𝑚2)

h Reflector depth(m)

h Heat transfer coefficient(W/𝑚2. 𝑘)

I Solar radiation (W/𝑚2)

k Thermal conductivity (W/m.K)

m Mass flowrate (kg/s)

n Number

Nu Nusselt number

P Power(kW)

Re Reynolds number.

t time(s)

T Temperature(℃, K)

U Heat transfer coefficient(W/𝑚2.K)

V Speed (m/s)

Greek letter

φrim rime angles

µ Viscosity

ατ Transmittance-absorption product

γ Shape factor

ε Emissivity

ξ Efficiency of heat recovery

ρ Density

ς Heat source

σ Stefan Boltzmann constant

Subscript

abs absorber tube

abs, i absorber, inner

abs, o absorber, outside

ap aperture

b beam

f working fluid

g glass cover.

L loss

o Overall

r Reflector width

r,abs-g Radiation between the absorber and glass cover.



Issn No : 1006-7930

Page No: 2072

r.g-sky Radiation between the glass cover and sky.

s sky

t total

u, 2 second unit

u,1 first unit

w Wind

Abbreviations

CHAPS Combined Heat And Power Solar

CSP Concentration Solar Power

DNI Direct Normal Insolation

DSG Direct Steam Generation

ET Evacuated Tubes

HTF Heat transfer Fluid

LFR Linear Fresnel Reflector

O&M Operation and Maintenance cost

PDC Parabolic Dish Collector

PI Parabolic Irradiant

PTCs Parabolic Trough Collectors

PTSP Parabolic Trough Solar Power

PV PhotoVoltaic

SPT Solar Power Towers

2. Experimental setup

The main components of the present test rig are concentrated solar collectors, Oil pump, Water pump,

three tanks, heat exchanger, connecting tubes, instruments, and fittings as shown in Fig (1). One axis tracking

concentrated solar collectors (the main part of the system) consists of two units of parabolic trough

concentrated solar collectors, each unit includes four parabolic trough collectors in series connection. A

stainless-steel reflector focuses the sunlight on a copper absorber tube of (12.25 mm) ID, (0.25mm) thickness.

The absorber is painted with a black selective coating, it is covered with a glass tube of (1500 mm) length, OD

(28mm) used to minimize thermal losses, and to accomplish the greenhouse effect. The collectors are mounted

by a rectangular iron structure of length (1500mm) and width (3120 mm) and achieved east-west single axes

tracking.

Fig. 1. Concentrated solar trough collectors.



Issn No : 1006-7930

Page No: 2073

Water pump MQS-126 is used to circulate water with flowrate range (10-30) lpm, between the heat

exchanger and water tank to cool the hot oil. It has specifications of head equal to (4-14) m, power of 0.37 W,

inlet diameter (ID)of (12.7mm), outlet diameter (OD) of (12.7mm). Oil pump (ZOT) is manufactured locally,

it consists of two parts, a motor in which three-phase power is converted to single-phase by converting the

binding from star to delta connection using a capacitor. The second part is a gearbox. It is used to circulate oil

(working fluid) with a maximum flow rate of (30 lpm) in the closed-loop system. It has specifications of head

equal to (5bar), power of(200W), inlet diameter (ID) equal to (12.7mm), outlet diameter (OD) equal to

(12.7mm). Three tanks are used in this work. The oil tank is used to supply oil to the solar concentrated trough

system, made of galvanized steel cubic shape with (700mm) height and (2mm) thickness and (620mm) width,

uninsulated, opened from top to atmosphere, and has one outlet from the bottom. Two horizontal cylindrical

water tanks manufactured from stainless steel, are connected together in series to supply water to the heat

exchanger. The length of the first tank is (1060 mm) and its diameter is (620 mm), the other has a length of

(960 mm) and diameter (520 mm). The finned helical tube is inserted in the water tank to form the heat

exchanger is used in this work. The cross-sectional area of the tank is of (1400mm) length and (400mm)

width, the tube has (12.5mm) outlet diameter and (1mm) thickness and (7620mm) length. Five ball valves are

used, have hollow-core work in the center of the body valve, with inlet diameter (12.7mm) and outlet diameter

(12.7mm). One for open and close discharge from the oil tank, one for a bypass to the oil tank, one for each

unit of concentrated solar collectors to control the oil flow rate in both branches, one for open and close

discharge from the water tank. And different parts of the closed-loop of the oil circulation are connected by an

insulated plastic tube. Different types of Instruments are used: Float meter model M-15G of range (0.5-5 gpm)

with other float meter model KTC S-25 of range (0.25-2.5𝑚3/h) are used at the inlet of each unit, to measure

the mass flow rate of the thermal oil. Forty-two K type thermocouples are used to measure the temperature at

different locations on the experimental setup. SIKA Boudren gages made from a stainless-steel material, are

used to measure the pressure at the inlet and outlet of each unit. Two Borden gages of range zero to 6 bar and

two Borden gages of range zero to 4 bar, with a resolution of (0.1 bar), are used in this work. Data of solar

radiation and wind speed for Baghdad city 33.3° North, are gained from Research Authority and Industrial

development / Energy and Environment Research Center, for the period (Extended from December 2018 till

May 2019).



Issn No : 1006-7930

Page No: 2074

Fig. 2. Schematic diagram of the connection of solar collector units. (a) parallel connection (b) series

connection.

Thermal oil is used as the heat transfer fluid, flowing through the absorber tube. The two collector units

are tests in parallel, and in series connection, as shown in Fig. (2).

2.1 Design consideration of parabolic trough reflector

A stainless-steel plate of (1 mm) thickness is formed the aperture width (700mm) and length (1500mm),

in parabolic shape according to [15]:

x2=4fy

(1)

where:

h =wr

2

16f

(2)

tanφrim

2=

wr

4f

(3)

f is the focal length (m)

wr is the reflector width(m)

h is the reflector depth(m)

φrim is rime angle, related to reflector width and focal length (deg). As viewed in Fig. (3)



Issn No : 1006-7930

Page No: 2075

Fig. 3. Schematic diagram of the reflector.

The design was based on enabling the solar collector to establish water (boiling), according to the solar

radiation received, in Baghdad-Iraq, in winter and summer. The considered parameters are environmental

conditions measurements, receiver length (Lr), thickness, inner diameter (di). Which are considered

as(1500mm,0.25mm,12.25mm) respectively.

The mass of heat transfer fluid (mr:kg) of density (ρ: kg/m3) enclosed inside the receiver is calculated as:

mr =π

4di

2Lrρ

(4)

The power required for water boiling (P: Watt) is evaluated as:

P = hfgmr

t

(5)

where:

hfg is water latent heat for boiling at atmospheric pressure (2257kJ/kg) [16].

t is time considered in this work (120 sec.)

Assume there is no heat loss, for the receiver in series connections for single collector unit, the total

power required is evaluated as:

P = Pi n collector

(6)

where:

Pi is the individual Power required for each receiver in the solar collector unit (W).

ncollector is Number of collectors (is equal to four in this work)

Pi = I Aap

(7)

In which:

I is Incident solar radiation. (W/m2)

Aapis aperture area of reflector (m2). It is calculated as:

Aap = Wr Lr

(8)



Issn No : 1006-7930

Page No: 2076

Wr is reflector width(m).

To calculate the required width of the reflector, the incident solar radiation for winter and summer seasons

must be figured. Considering water as heat transfer fluid, the following calculations are followed to ensure

water boiling at atmospheric. The power required for water phase change in:

Summer season

The density of water at temperature 30 ℃ (ρ) is 995.10 kg/m3 .According to Eq. (A-5) in the appendix, so

the water mass is 0.175 kg. According to Eq. (4).

The power required for boiling (P) is 3291.454 W. According to Eq. (5), and the individual Power

required (Pi) is 822.86 W.

The reflector aperture area is evaluated based on maximum solar radiation in the summer season of I =

1200W/m2, such that Aap= 0.685 m2. Which results from reflector width of: 𝐖𝐫 = 0.457 m.

While in Winter season

The density of water at temperature 10 ℃ (ρ) is 999.77 kg/m3. According to Eq. (A-5) in the appendix, so

the mass of water is 0.176 kg. According to Eq. (4).

The power required for boiling (P) is 3310.27 W. According to Eq. (6), and the individual Power required

(Pi) 𝑖𝑠 827.56 W.

The reflector aperture area is evaluated based on minimum solar radiation in the winter season of I =

700W/m2, such that Aap= 1.18 m2. Which results from reflector width of: 𝐖𝐫=0.78 m.

Since higher aperture area collects higher values of solar radiations, reflector width of (0.78 m) is adopted

in the present work.

See us, use oil instead of water as HTF, as a results of the evaporation of water and change in the

amount of mass passing through the absorber tube when change the connection to series, and the ability of an

oil to reach temperature above boiling temperature.

2.2. Collector theoretical analysis

The solar radiation is absorbed by a glass cover and the absorber receiver, while the thermal loss is from

glass cover, due to convection and radiation from glass cover to the surrounding, as shown in Fig. (3).

Fig. 4. Thermal resistance network and energy balance for the solar receiver.



Issn No : 1006-7930

Page No: 2077

The collector efficiency factor (F′) is the ratio of the actual useful energy to the collected useful energy if

the entire absorber surface is at the mean fluid temperature [17]. It is calculated by:

F′=Uo

UL (11)

where:

UL is the heat loss coefficient (W/m2.K)

Uois the overall heat transfer coefficient (W/m2.K) is calculated by ⦋ 17⦌:

Uo = ⦋1

UL+

Dabs,o

hf,iDabs,i+

Dabs,oln(Dabs,oDabs,i

)

2kabs⦌−1 (12)

In which:

Dabs,o is the outside diameter of the absorber ( m).

Dabs,i is the inside diameter of the absorber ( m).

kabsis the thermal conductivity of copper (385 W/m. ℃). [18].

hf,i is the heat transfer coefficient inside absorber, it is calculated as:

hf,i=Nufkf

Dabs,i (13)

In which:

kf is the thermal conductivity of oil based on inlet

temperature(W/m.℃), it is calculated by Eq. (A-1), appendix.

Nuf is the Nusselt number of the working fluid. The value of Nusselt number is equal to 4.36 according to [18],

when Reynolds number is lower than 2300 (laminar flow), and exposed to constant heat flux.

Reynolds number (Ref) value is based on the internal diameter of the absorber; it is expressed as:

Ref =4mf

°

π D abs,i µf (14)

In which:

mf° is the mass flow rate of oil (kg/s).

µf is the dynamic viscosity of the working fluid based on inlet temperature. It is calculated by Eq. (A-2),

appendix.

In this work, the adopted mass flow rate of oil is (0.03, 0.06, 0.07, 0.09, 0.10, 0.12, 0.13) kg/s, and the

corresponding Reynolds Number Obtained at solar noon is ranged between (448.55, 879.24, 1110.16,

1441.76, 1465.05, 1836.13, 2216.76) i.e. laminar flow.

The heat loss coefficient (UL) combines thermal losses in just one coefficient, it is calculated by ⦋ 17⦌

UL = ⦋ Aabs

(hw+hr,g−sky)Ag

+1

hr,abs−g ⦌−1 (15)

where:

Aabs is the absorber surface area (m2).

Ag is the area of the glass cover. (m2)

hw is the convective heat transfer coefficient (W/m2.K) due to wind speed, it is calculated as [4]:

hw=4 Vw0.58 Dog

−0.42 (16)

Vw is the wind speed (m/s), and Dogis the outside diameter of the glass cover (m).

hr,g−sky is the Heat transfer coefficient between the glass cover and sky, due to radiation (W/m2. K). It is

calculated as ⦋ 17⦌

hr,g−sky = εgσ(Tg + Ts)(Tg2 + Ts

2) (17)

In which:

𝜀𝑔 is the emissivity of the glass cover (0.82). [19].

𝜎 is Stefan Boltzmann constant (5.67× 10−8W/m2.K4)



Issn No : 1006-7930

Page No: 2078

Tg is the glass cover temperature (K).

Ts is the sky temperature (K), it is calculated as [17]:

Ts=0.0552 Tamb1.5

(18)

Tamb is the ambient temperature (K).

hr abs−g is the heat transfer coefficient between the absorber and glass cover, due to radiation (W/m2.K). It is

calculated by [17]

hr,abs−g= σ(Tabs+Tg)(Tabs

2 +Tg2)

1

εabs+

AabsAg

(1

εg−1)

(19)

where:

εabs is the emissivity of the absorber metal (0.05) [19]

Tabsis the absorber surface temperature (K).

The heat removal factor (𝐹𝑅) is the ratio of the actual useful energy collected to that would be collected

if the entire absorbed surface is at the inlet fluid temperature. It is determined as ⦋20⦌

FR =mf

°cp

AapUL⦋1 − e

(−AapULF′

m°fcp

)⦌ (20)

where:

Cp is the specific heat of oil (kJ/kg. K) based on inlet temperature. It space from eq (A-3), appendix A.

Aapis the aperture area (1.05 m2)

The collector geometry factor (F″) is the ratio of the heat removal factor to the collector efficiency

factor. It is calculated as ⦋ 17⦌

F″=FR

F′ (21)

2.3 Concentrated solar collectors’ efficiency

Thermal efficiency (ηth) of the concentrated solar collectors is the ratio of collector heat gain to incident

solar radiation times aperture area. It depends on operating conditions such as inlet oil temperature, solar

radiation, ambient temperature, and wind speed. Several parameters such as the collector efficiency factor,

the heat removal factor, and the collector geometry factor also affect thermal efficiency. The thermal

efficiency of each collectors’ unit is defined as:

ηth =Heat gain

Ib Aap NO.of reflectors (22)

The useful heat (Qga) gain by oil flow through the absorber [20], it is calculated by the equation:

Qga= mf° CpΔT (23)

where:

Ib is incident beam solar radiation (W/m2).

The optical efficiency of the solar receiver represents the ratio of absorber grasped energy to that incident

on the receiver [16] such that:

ηopti= G

Ib (24)

In which

G is the actual amount of absorbed energy by the receiver. It is computed such as [17]

G=Ib( ρoτgαaγ)k(θ) (25)

where:

ρo is reflector surface reflectivity (0.95).

𝜏𝑔 is glass transmissivity (0.84).



Issn No : 1006-7930

Page No: 2079

𝛼𝑎 is absorber absorptance (0.905).

γ is the shape factor of the receiver due to inexact orientation (0.9).

k(θ) is the incident angle modifier. It is calculated as [21]:

k(θ) = 1 −f

Lr(1 +

Ap2

48 f2) tan θ (26)

In which :

Aapis the aperture area (1.05 m2)

f is the focal length (0.14 m).

Lr is the collector length (1.5 m).

θ is the incident angle (0°).

The present collectors tracked solar radiation with day time so the incident angle approaching 0, results an

incident angle modifier approaching 1, according to eq.

Table (1) Specifications of trough solar collectors

Solar collector

elements

description

Value

glass tube Outer diameter (Dg,o), m 0.28

Inner diameter (Dg,i),m 0.25

Length (L)m 1.5

area (Ag) m2 = πDL 0.13188

Copper absorber tube Outer diameter (Dabs,o), m 0.01250

Inner diameter (Dabs,i),m 0.01225

area (Aabs), m2 = πDabs,oL 0.058875

Stainless steel

Reflector

Length (Lr), m 1.5

Reflector width (Wr), m 0.7

Aperture area (Aap), m2 1.05

Concentration ratio (Cr) =Aap

Aabs 17.83

2.4 Error Analysis

The uncertainty of experimental results is, due to the accuracy of the instruments and human errors. So,

the uncertainty of thermal efficiency can be affected by the accuracy of the parameters, given in table (2).

Table (2): Accuracy of the parameter used in the error analysis calculated

Independent

variables

Temperature

(℃)

Solar radiation

(W/𝒎𝟐)

Specific heat

(J/kg. K)

Length

(m)

Mass flow rate

(kg/s)

Uncertainty interval ±0.1 ± 10 𝑊/𝑚2 ±0.2 0.001 0.00236

The certainty can be evaluated, according to [17] the percentage error of the collector efficiency is

calculated as:

wR = ⦋(∂R

∂x1w1)

2+ (

∂R

∂x2w2)

2+ ⋯ + (

∂R

∂xnwn)2⦌

12⁄ (27)



Issn No : 1006-7930

Page No: 2080

wη = ⦋(∂η

∂ΔTwΔT)

2+ (

∂η

∂IbwIb

)2

+ (∂η

∂m⃘wm⃘)

2+ (

∂η

∂LrwLr

)2⦌1

2⁄ . (28)

where

wR: The uncertainty in the result

wn : The uncertainty in the independent variables.

For parallel connection:

wη

η =7.8%

For series connection:

𝑤𝜂

𝜂 = 4.2 %

3. Collector connection type

Parallel and series connections for the two units of concentrated solar collectors are tested in this work.,

as shown in Fig (2) a and b.

3.1 Parallel Connection

The total heat gain (Qga,p) of two units in parallel connection is equal to:

Qga,p=Qu,1 + Qu,2= mf° CpΔT (29)

where:

Qu,1 is the heat gain of the first unit.

Qu,2 is the heat gain of the second unit.

In this case the mass flow rate ( mf° ) of the system is whole doubled the mass flow rate of each unit,

but the temperature difference (ΔT) between oil inlet and outlet is the same in each unit.

mf° = m1

° + m 2° (30)

where:

m1 ° is the mass flow rate of the first collector’s unit (kg/s).

m 2° is the mass flow rate of the second collector’s unit (kg/s).

Thermal efficiency (ηth) of the system, in this case, is equal to

ηth =Qga,p


3.2 Series Connection

The total heat gain (Qga,s) of two units in series connection is equal to:

Qga,s=Qu,1 + Qu,2= mf° CpΔT (32)

In this case the oil mass flow rate (mf°) of the concentrated solar collector's system is the same in each

unit, but the temperature difference (ΔT) between oil inlet to system and outlet is equal to the summation of

the value of each concentrated solar collector's unit.

ΔT = ΔTu,1 + ΔTu,2 (33)

where:

ΔTu,1 is the temperature difference for the first collector’s unit (℃).

ΔTu,2 is the temperature difference for the second collector’s unit(℃).

Thermal efficiency (ηth) of the system, in this case, is equal to

ηth =Qga,s




Issn No : 1006-7930

Page No: 2081

4. Experimental procedure

The experiments are carried out during a clear sky day at Baghdad, Iraq. All reflectors and glass cover

are cleaned, then all instruments are checked. Control valves are opened, and the oil pump is turned on the

collector units are tested under different connections (series and parallel), for five mass flow rates of oil for

each connection. All readings are considered every 10 minutes.

5. Results and discussions:

Two types (parallel, series) of connection were tested. The tests started at 8;30 am consecutively for

each flow rate.

5.1 Ambient Conditions

Fig. (5) (a,b) present the ambient temperature, wind speed, and solar radiation, show that the ambient

temperature in both days (30th Dec, and 5thJun) are similar in their behavior, but they are a difference in their

values, they are starting from (12, 14) ℃ in a, b respectively, then its values increased until it reaches the

higher value at noon, then decreased its value. Wind speed value (13 km/hr.) is the maximum in both days

(a,b), while the minimum is( 5,9) km/hr. respectively in a and b. Solar radiation rises from(173,234)W/m2 at

8:30 am to reach its maximum value at the noon respectively in a and b to (628, 719) W/m2, there is a

similarity in climatic conditions almost for these two days.

Fig. 5. Weather condition for selected clear sky day (a) 30𝑡ℎ Dec. 2018 (parallel connection) (b) 5𝑡ℎ Jun.2019

(series connection).

5.2 Pressure drop

Fig. (6) presents pressure drops with time for 30 𝑡ℎ 𝐷𝑒𝑐, for two units of solar collectors connected in a

parallel connection. And 5th Jun for two units of solar collectors connected in a series connection. The

pressure almost oscillates at the beginning then stabilizes, as a result, the system contains air at the start of the

operation.

Fig. (7) presents pressure drops with Reynold number, 30 𝑡ℎ 𝐷𝑒𝑐, for two units of solar collectors

connected in a parallel connection. And 5th Jun for two units of solar collectors connected in a series

connection. The pressure drops increased with the Re number increased.



Issn No : 1006-7930

Page No: 2082

Fig. 6. pressure drop with time for (30 th Dec, and 5th Jun)

the selected clear sky day

.

Fig. 7. pressure drop with Re number for different hydraulic thermal oil mass flow rates

0.0294, 0.0442, 0.05165, 0.0591, 0.0738, 0.0959) kg/s in parallel connection, and

(0.0584, 0.07325, 0.088, 0.117515, 0.1386) kg/s in series connection, for selected

clear sky day.

5.3 Temperature history

Fig. (8.a) presented inlet temperature history of oil (heat transfer fluid), for each receiver of unit I when

the two units are connected in a parallel, for 2 lpm oil flowrate in each absorber, it has a maximum

temperature of oil inlet is equal to 63.9 ℃ for the fourth absorber tube at noon, because temperature of oil

continued to increase due to continued exposure to heat. Fig (8.b) shows the oil outlet temperature from each

absorber of unit I. The maximum achieved temperature is equal to 76.2 ℃ for the fourth absorber tube at noon



Issn No : 1006-7930

Page No: 2083

because at this time has solar radiation higher than the rest of the day. Fig. (8.c) shows the inlet temperature

history of oil (heat transfer fluid), for each receiver of unit II when the two units are connected in a parallel,

for 2 lpm oil flowrate in each absorber, it has a maximum temperature of oil inlet is equal to 62.9℃ for the

fourth absorber tube at noon. Fig. (8.d) shows the oil outlet temperature from each absorber of unit II. The

maximum achieved temperature is equal to 74.9℃ for the fourth absorber tube at noon because the

temperature of oil continued to increase due to continued exposure to heat, and at this time has solar radiation

higher than the rest of the day. Observe asymmetry in the distribution of temperatures between the two units,

due to their exposure to the same climatic conditions.

Fig. 8. Oil temperature history for solar collector unit in parallel connection on 30𝑡ℎ Dec. for (2 lpm) (a)inlet

to

each receiver in unit I (b)outlet from each receiver in-unit I. (c)inlet to each receiver in unit II. (d) outlet from

each receiver in unit II.

Fig. (9.a) presented inlet temperature history of oil (heat transfer fluid), for each receiver of unit I, II

when the two units are connected in a series, for 4 lpm oil flowrate in each absorber, it has a maximum



Issn No : 1006-7930

Page No: 2084

temperature of oil inlet is equal to 73.6 ℃ for the 8𝑡ℎ absorber tube at noon. Fig (8.b) shows the oil outlet

temperature from each absorber of unit I, II. The maximum achieved temperature is equal to 80.3℃ for the

8𝑡ℎ absorber tube at noon, because the temperature of oil continued to increase due to continued exposure to

heat. And at this time has solar radiation higher than the rest of the day.

Fig. 9. Oil temperature history for solar collector units on 5𝑡ℎJun. For (4 lpm) series connection. (a) the inlet

to

each receiver (b) outlet from each receiver.

5.4 Heat gain history

Fig. (10) presented heat gain history for each units connection, it is viewed that the series connection

has thermal performance better than a parallel connection, and the maximum heat gain was (4428.8, 5145.4)

W/m2 for parallel connection and series connection respectively, because of that the series connection has a

higher heat absorption than the parallel connection.

Fig.( 11) presented the heat gain of collector system, and the temperature difference between the inlet

and the outlet of the two collector units I, II when the two units are connected in a series, for different

hydraulic thermal oil mass flow rates (5, 6, 7 ,8 ,9) lpm. The maximum temperature difference is 45.3 ℃ and

maximum heat gain is 5424.87 W/m2 with mass flow rate equal to 5 lpm.



Issn No : 1006-7930

Page No: 2085

Fig. 10. Heat gain various with time for parallel and series connection.

Fig.11 Heat gain and temperature difference history for solar collector units various with a flow rate of oil for

series connection.

5.5 Heat Removal Factor

Fig. (12) (a, b) presented Heat removal factor history, for each receiver of unit I, II when the two units

are connected in a parallel, for 2 lpm oil flowrate in each absorber, show that the value of the heat removal

factor of each collector has little difference from others, although flow the same flow rate of oil in each one.

Fig. (5-12) (c, and d) presented Heat removal factor history, for each receiver of unit I, II when the two units

are connected in a a series, show that the value of heat removal factor of the collector in (c, and d) is higher

than in (a, and b), due to the flow rate of oil in system with series connection is 4lpm higher than 2 lpm with

parallel connection.



Issn No : 1006-7930

Page No: 2086



Issn No : 1006-7930

Page No: 2087

Fig.12 Heat removal factor for solar collector system varies with time, for parallel connection: (a) unit I (b)

unit II. and for series connection: (c) unit I (d) unit II.

5.6 Thermal Efficiency history

Fig. (13), (14) present thermal efficiency history for the two units of solar collector system, with two

connections (parallel, series).

Fig. (13) The present thermal efficiency history of each connection, the value of it is equal to (0.839,

0.851) respectively for the parallel connection, series connection. This figure shows that the series connection

has thermal efficiency better than the parallel connection all the time because the series connection has heat

gain higher than a parallel connection.

Fig. (14) The present thermal efficiency history of each connection varies with calculated inlet

temperature minus ambient temperature divided by incident solar radiation. 30 th Dec, for two units of solar

collectors connected in a parallel connection. 𝐴𝑛𝑑 5𝑡ℎ 𝐽𝑢𝑛 for two units of solar collectors connected in a

series connection. The thermal efficiency of two connection decrease with the values of calculated inlet

temperature minus ambient temperature divided by incident solar radiation increased.

Fig. 13. Efficiency various with time.



Issn No : 1006-7930

Page No: 2088

. Fig. 14. Efficiency for various inlet temperature levels for parallel and series connection.

6. Conclusions

Two units of concentrated solar collectors are manufactured, and tested in (parallel and series) connection,

to investigate the better thermal performance obtained, after comparing the system performance with each

connection. Found:

I. The absorber surface temperature increases with the ward of oil flow, so the oil temperature flow

inside the absorber is increased.

II. Different amounts of oil flow rates inside the absorber are taken, show that the increase in the mass

flow rate of oil decreases the heat transfer between absorber and heat transfer fluid.

III. The maximum oil temperature outlet from two-unit in parallel connection is equal to 63.9 ℃, for solar

radiation equal to 628 W/m2, and the maximum oil temperature outlet from two-unit in series

connection is equal to76.2 ℃ for solar radiation equal to 719 W/m2, so the oil temperature with series

connections is higher than with parallel connections.

IV. The maximum efficiency of two units in parallel connection is equal to 0.839 and the maximum

efficiency of two-unit in series connection is equal to 0.851, so the system with series connections has

efficiency is higher than with parallel connections.

V. A series connection is better thermal performance.

VI. Improve the daily thermal efficiency of the two units by 1.42%.

VII. Increase the heat gain from 4468.4 W/m2. in parallel connection to 5154.94 W/m2. in series

connection.

Acknowledgment

We gratefully Appreciate to the Ministry of Higher Education and Scientific Research Unversity of Baghdad

for support them.



Issn No : 1006-7930

Page No: 2089

Appendix.

Thermophysical properties of circulating fluid(oil) are calculated as:

Thermal conductivity (W/ m. K)

k = - 1× 10−20T3 + 3× 10−7T2 - 1× 10−4 𝑇 + 0.1471

(A-1)

Viscosity (Pa. s)

µf =6.672× 10−13T4- 1.566× 10−9T3+ 1.388× 10−6T2- 5.541× 10−4+8.487× 10−2

(A-2)

Specific Heat (J/kg℃)

Cp=1.708T+1107.798

(A-3)

Density (kg/𝐦𝟑)

ρ = - 3× 10−17T3 + 6 × 10−16T2 – 0.6 T + 900.2

(A-4)

While that for water are:

ρ =- 1× 10−5 T3 - 56 × 10−4 T2 + 0.0037 T + 1000.3

(A-5)

References

[1] Coventry J. S., 2005,” Performance of a concentrating photovoltaic/thermal solar collector”, Solar Energy, 78,

211-222.

[2] Morin G., Dersch J., Platzer W., Eck M. and Haberle A., 2012,” Comparison of linear Fresnel and Parabolic

trough collector power plants”, Solar Energy, 86, 1-12.

[3] Zhang H. L., Baeyens J., Degreve J. and Caceres G., 2013,” Concentrated solar power plants: Review and

design methodology’, Renewable and Sustainable Energy Reviews, 22, 466-481.

[4] Bellos E., Tzivanidis C. and Tsimpoukis D., 2017,” Thermal enhancement of parabolic trough collector with

internally finned absorbers”, Solar Energy, 157, 514-531.

[5] Nasir K. F., 2019,”Thermal performance of plastic Receiver in solar collector”,University of Baghdad, Journal

of Engineering N.7, V. 25.

[6] Syuhada A., Maulana M. I., Adria A., 2019, “Experimental Analysis of The Effect of Tube Pass Length on

The Characteristics of Heat Transfer in V-Sharp Turned Channels”, Journal of Advanved Reasearch in fluid

Mechanics and Thermal Sciences, 124-131.

[7] Bakos, G. C. 2006,” Design and construction of a two-axis Sun tracking system for parabolic trough collector

(PTC) efficiency improvement”, Renewable Energy, 31, 2411-2421.

[8] Qu W., Wang R., Hong H., Sun J. and Jin H., 2017,” Test of a solar parabolic trough collector with rotatable

axis tracking”, Applied Energy, 207, 7-17.

[9] Riffelmann K. J., Neumann A. and Ulmer S., 2006,” Performance enhancement of parabolic trough collectors

by solar flux measurement in the focal region”, Solar Energy, 10, 1303-1313.

[10] Figueredo S. L., 2011,” Parabolic trough solar collectors: Design for Increasing Efficiency”, Massachusetts

Institute of Technology,

[11] Fernandez-Garcia A., Zarza E., Valenzuela L. and Perez M., 2010, "Parabolic- trough solar collectors and

their applications”, Renewable and Sustainable Energy Reviews, 14, 1695-1721.

[12] Hoseinzadeh H., Kasaeian A. and Shafii M. B., 2018,” Geometric optimization of parabolic trough solar

collector based on the local concentration ratio using the Monte Carlo method”, Energy Conversion and

Management, 175, 278-287.



Issn No : 1006-7930

Page No: 2090

[13] Xu L., Sun F., Ma L, Li X., Lei D., Yuan G., Zhu H., Zhang Q., Xu E. and Wang Z., 2019,” Analysis of

optical and thermal factors’ effects on the transient performance of parabolic trough solar collectors”, Solar

Energy, 179, 195-209

[14] Xu L. C., Liu Z. H., Li S. F., Shao Z. X. and Xia N., 2019,” Performance of solar mid-temperature evacuated

tube collector for a steam generation”, Solar Energy, 183, 162-172.

[15] Mohamed E. A., (2013),” Design and testing of a solar parabolic concentrating collector “, International

conference renewable energies and power quality, V.XXX, P.XXX-XXX.

[16] Joseph H. K., Frederick g. k., Philip G. H., and Joan G. M., “Thermodynamic properties of water including

vapor, liquid, and solid phases”, JounWily &son Pub.

[17] Duffie J. A. and Beckman W. A., (2013),” Solar Engineering of thermal process”, Wiley Inter-science

Publications, John Wiley and Sons, New Jersey.

[18] Holman E. W., (1989),” Some evolutionary correlates of higher taxa. Paleobiology”, V.15, PP.357-363.

[19] Gengel Y. A., 1998,” Heat Transfer: A practical approach “, Tom Kasson, New York.

[20] Kahrobaian A. and Malekmohammadi H., (2008),” Exergy Optimization Applied to linear parabolic Solar

collectors”, Journal of Faculty of Engineering, V.42, PP.131-144.

[21] Mokhtari A., Yaghoubi M., Kanan P., Vadiee A., and Hessami R., 2007, “Thermal and optical study of

parabolic trough collectors of shiraz solar power plant”, Thermal Engineering: Theory and Applications.



Issn No : 1006-7930

Page No: 2091

performance evaluation of concentered trough solar

Documents