Parabolic Trough Solar Power Optimization in Bangladesh

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Performance optimisation of parabolic trough solar thermal power plants – a
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Performance optimisation of parabolic trough
solar thermal power plants – a case study in
Bangladesh
Noushad Bhuiyan, Wali Ullah, Rabiul Islam, Tofael Ahmed & Nur Mohammad
To cite this article: Noushad Bhuiyan, Wali Ullah, Rabiul Islam, Tofael Ahmed & Nur Mohammad
(2019): Performance optimisation of parabolic trough solar thermal power plants – a case study in
Bangladesh, International Journal of Sustainable Energy, DOI: 10.1080/14786451.2019.1649263
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Performance optimisation of parabolic trough solar thermal
power plants – a case study in Bangladesh
Noushad Bhuiyan, Wali Ullah, Rabiul Islam, Tofael Ahmed and Nur Mohammad
Department of Electrical and Electronic Engineering, Chittagong University of Engineering and Technology,
Chittagong, Bangladesh
ABSTRACT
In this paper, parabolic trough solar thermal plants are proposed. Two key
parameters, solar multiple (SM) and full load hours of thermal energy
storage (TES), are optimised by maximising the annual energy and
minimising the levelized cost of electricity (LCOE). This work presents a
comparative study, based on the 4E (energy–exergy–environment–
economic) analysis, of the optimised plant with a reference plant. The
performance evaluation of the optimised plant is carried out in eight
different locations of Bangladesh. The result obtained shows that the
salt plant has the best performance compared to that of the thermal oil
plant. The salt plant requires a lower LCOE of 9.86 ¢/kWh. It also
possesses a maximum capacity factor of 39.5% and generates the
highest annual energy of 171.1 GWh. From the feasibility study, Cox’s
Bazar is found as the best location of eight different regions of Bangladesh.
KEYWORDS
Concentrated solar power
(CSP); heat transfer fluid
(HTF); LCOE; Parabolic
trough; Solar multiple;
thermal energy storage (TES)
Nomenclature
A Aperture area of the collector (m2
) PGnet Net power generation (kW h)
B Hourly direct solar irradiance on a horizontal
surface (kW/m2
)
Q̂back Thermal energy supplied in FBS (kW h)
Bo Daily extra-terrestrial irradiance (kW/m2
) Q̂total Totalenergyneededtoreachthethermodynamicstate(kWh)
C Construction year Q̂incident Total incident solar energy received by collector aperture
area (kW h)
CONc Construction schedule Q̂received Total thermal energy received by PB (kW h)
D Hourly diffuse solar irradiance on a horizontal
surface
Q̂thermal SF Thermal power attained by the solar field at design point
(kW h)
Dn Number of days in a year Q̂thermal PB Thermal power required by the power block at nominal
conditions (kW h)
dr Relative earth-sun distance Sd Sunshine duration (hr)
Estorage TES thermal capacity (kW h) SMdp Solar multiple at the design point
Êxrecived SF Exergy received by solar field (kW h) SSF outlet Entropy at the outlet of solar field (kJ/kg °C)
Êxuseful Useful exergy supplied by the receiver (kW h) SSF inlet Entropy at the inlet of solar field (kJ/kg °C)
ÊxSF0 Exergy at the outlet of solar field (kW h) Sd max Maximum possible sunshine duration
ÊxSFi Exergy at the inlet of solar field (kW h) Tambient Ambient temperature (K)
Êxrecived PB Exergy received by power block (kW h) wdct Design cycle thermal equivalent (kW)
F Dilution factor Tsun Temperature of sun (K)
fback Fossil fill fraction hcycle Cycle efficiency at design point
G Hourly global solar irradiance on a
horizontal surface
hen SF Solar field energy efficiency
H0 Daily diffuse irradiance on horizontal
surface (kW/m2
)
hen PB Power block energy efficiency
HD Daily diffuse irradiance on horizontal surface
(kW/m2
)
hex SF Solar field exergy efficiency
© 2019 Informa UK Limited, trading as Taylor & Francis Group
CONTACT Nur Mohammad nur.mohammad@cuet.ac.bd
INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY
https://doi.org/10.1080/14786451.2019.1649263

HG Daily global irradiance on horizontal surface hex PB Power block exergy efficiency
hin tur Max turbine over design operation hex overall Overall exergy efficiency
hout SF Min turbine operation hen overall Overall energy efficiency
hSF0 Enthalpy at outlet solar field (kJ/kg) vs Sunrise hour angle
hSFi Enthalpy at inlet solar field (kJ/kg) d Solar declination
I Hourly direct normal irradiance w Latitude (°)
Ib Solar irradiance on aperture area (kW/m2
) Dn Total number of desired storage hours (hr)
m̂ Turbine out fraction m̂SF Mass flow rate of the HTF in the SF (kg/s)
m̂f Estimated Gross to Net Conversion Factor N Analysis period
1. Introduction
Fossil-fuel reserves are shrinking day by day due to the excessive energy use. This results in a signifi-
cant attention to renewable energy sources throughout the world. Among the alternative renewable
energy resources, solar energy is the most recognised one since it is unlimited and can easily be con-
verted into electricity (Mohammad and Mishra 2017). Moreover, solar energy is eco-friendly and
free. Solar energy can be harnessed in two ways. One is solar photovoltaic, and another is solar ther-
mal, which is generally known as concentrated solar power (CSP). Power extracted from CSP tech-
nology is reliable, clean and environmental friendly (Quamruzzaman et al. 2016). In this study, we
investigate the performance of the CSP based power plant in the context of Bangladesh.
Bangladesh is one of the largest deltas in the world. It has a significant variation of topographic,
climatic and socio-economic characteristics owing to its geographical location (Hasan and Moham-
mad 2019; Hossain et al. 2019). It is one of the most densely populated countries with a rapid growth
rate over the last 100 years (Faisal and Parveen 2004). Per capita energy consumption is very much
lower than that of the other part of the world, and in the year 2014, it was 311 kWh (Islam and Khan
2017). According to the Bangladesh Petroleum Statistical Review of World Energy, total energy gen-
eration in Bangladesh is 67.4 TWh. This energy demand is supplied mostly from fossil-fuel resources
such as natural gas, imported oil and coal. Burning these fossil-fuels produce 78.5 metric tons of
CO2. In addition, the energy consumption growth rate is increasing day-by-day due to rapid econ-
omic progress and population growth. Increasing energy consumption results in the escalation of
CO2 emission (Mohammad and Mishra 2018a, 2018b). Climate change and other environmental
concerns led the Bangladesh government to the adoption of an energy policy encouraging rapid
uptake of solar energy (Mohammad and Rahman 2019). Several solar energy pilot projects have
been initiated to increase the proportion of renewable energy in the mainstream power sector (“Ban-
gladesh Power Development Board,” 2017). Due to the geographic location in the tropical zone, Ban-
gladesh is blessed with huge amount of solar irradiance. However, the potentiality of electricity
generation from the CSP is still untapped.
The CSP technology mainly includes four alternatives namely parabolic trough, solar tower, linear
Fresnel, and parabolic dish. For utility-scale power generation, parabolic trough technology is one of
the most matured and proven technology in dry and semi-arid regions (Boukelia and Mecibah 2013).
It concentrates sunlight on a single focal point by using several parallel curved mirrors. A receiver
tube containing Heat Transfer Fluid (HTF) is used to convert the sunlight into electricity. It may
range from a few kilowatts for a remote power system to hundreds of megawatts for a grid-connected
system (Boukelia et al. 2015).
Parabolic trough CSP technology mainly consists of storage system, solar field area, and power
cycle. A fossil back-up system for increasing the plant reliability has been investigated in (Zhang
et al. 2013). In the past, parabolic trough technology based on synthetic oil and water as working
fluids was used. But at present, molten salt is used as an HTF and storage medium (Dunn, Hearps,
and Wright 2012). The main disadvantage of using thermal oil as HTF is the highest temperature
limitation in the solar field area.
The Levelized Cost of Electricity (LCOE) is a key parameter for viability study of CSP plants. The
LCOE varies with different parameters such as working fluid, plant configuration, solar radiation,
2 N. BHUIYAN ET AL.

plant performance, investment cost, and running costs of the plant. Continuous research is in pro-
gress for reducing the LCOE and increasing the annual generation of electricity.
Llorente García, Álvarez, and Blanco (2011) developed a 50 MW parabolic trough solar power
plant model and investigated the performance with full load hours. Thermal Energy Storage
(TES) and a Therminol VP-1 in the solar field as HTF was used. The simulation results were com-
pared with a thermal power plant functioning in Spain. Montes, Abánades, and Martínez-Val (2009)
studied the effect of solar multiple (SM) on LCOE, annual energy generation, and natural gas com-
bustion. The plant was integrated with auxiliary natural gas-fired boiler and TES system. Feasibility
study of CSP technology in different regions of Bangladesh has been studied by Islam, Bhuiyan, and
Ullah (2017). Kearney et al. investigated how to enhance system performance and minimise cost of a
molten salt based CSP with TES medium in a parabolic trough system (Kearney et al. 2004). The
result of that study favours the use of molten salt rather than synthetic oil as an HTF and storage
medium since it maximises the annual energy generation and minimises the LCOE. A year-round
optimisation process of a molten salt-based CSP technology is presented by Martín and Martín
(2013). The cost-benefit analysis of the parabolic trough thermal power plant to produce power
in the arid and semi-arid region is studied by Poullikkas (2009). In that study, they investigated
different parameters affecting economy such as full load hours, capital cost and capacity factor to
identify the least cost. A thermodynamic model of a 100 MW plant to evaluate the performance
of a hybrid parabolic trough system with fossil fuel backup is presented by Larraín, Escobar, and Ver-
gara (2010).
Kalogirou (2013) developed a 50 MW parabolic trough thermal power plant to minimise LCOE,
considering the required land area and several technical characteristics. The study compares four
types of CSP technology and suggests that parabolic trough best suits for a large-scale power gener-
ation. An energy-efficient economic model of a 100 MW parabolic trough solar power plant is pre-
sented by Abbas et al. (2013). The model uses molten salt as HTF which reduces the LCOE.
Boukelia et al. (2015) performed the feasibility study of parabolic trough-based CSP for different
locations in Algeria by considering four factors: energy, environment, exergy, and economy (4E).
The study presents an optimisation based on parametric analysis of SM, TES with a full load, and
the LCOE. However, their work lacks the techno-economic performance analysis of the CSP consid-
ering some of the influential factors such as solar radiation, ambient temperature, and key design
parameters. To address this research gap, this paper proposes an optimisation model for CSP in
the context of Bangladesh considering the parameters above which leave in T E Boukelia et al. as
mentioned above. This model would improve the accuracy of the obtained result.
This work aims to optimise parabolic trough CSP (PTCSP) and to identify suitable placement for
implementing the PTCSP in Bangladesh. After optimising the PTCSP, a comparative study is done
with the model in Boukelia et al. (2015) as the reference plant.
Organisation of the rest of the paper is as follows: Section 2 provides the Methodology of the
Power Extraction from the PTCSP. Results and Discussions are presented in Section 3 followed
by Conclusions in Section 4 at the end.
In summary, prime objectives of the current work are: (i) To optimise the PTCSPs using molten
salt and synthetic oil as HTF. The plant is integrated with full load hours of TES and solar multiple.
(ii) To compare the performance between the two PTCSPs taking the 4E into account. (iii) To ana-
lyse the viability of the plant that emerges from the 4E study for eight different suitable locations in
Bangladesh.
2. Methodology
Different configurations of the PTCSP are available in the literature mainly reliant on a number of
factors like solar field area, HTF, power block cycle and so on. The configuration depends on the
absence or presence of some auxiliaries like fuel backup and TES. The 4E analysis of CSP requires
the selection of effective and reliable model configuration. The geographical and weather data along
INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 3

with various criteria parameters are accessed from the System Adviser Module (SAM), which is
developed by NREL (“System Advisor Model (SAM),” 2017). SAM is useful for performance predic-
tion and cost of energy estimation. (“System Advisor Model (SAM),” 2017). The methodology of this
study is shown in Figure 1 and it consists of the following steps:
. Collecting meteorological, economic and technical data from SAM.
. Simulating and optimising two plants, the first one is by using synthetic oil, and the second one is
using molten salt with varying SM and full load hours TES.
. Comparing, based on 4E (energy–exergy–environment–economic), among the optimised plants
to select the most suitable PTCSP.
. A feasibility study, with the selected PTCSP, to find out the most favourable location of eight
different regions in Bangladesh.
2.1. CSP plant’s configurations
This study considers two types of PTCSP, which are integrated with fuel backup system and thermal
electric storage. The plants are configured with two types of heat transfer fluids namely Therminol
VP-1 and Hitec solar salt. The inlet and the outlet of the solar aperture area temperature ranges from
296°C to 393°C for Therminol VP-1 and from 293 °C to 550 °C for Hitec solar salt, respectively.
These two conventional HTF fluids are frequently used in CSP based power generation system
(Giostri et al. 2012). Major input parameters are summarised in Table 1. There are two types of gen-
eric collectors in the solar field area. One type is ‘Siemens SunField 6’, which we used for synthetic oil
plant, and the other type is ‘SkyFuel SkyTrough (with 80-mm OD receiver)’, which is used for molten
salt plant. The ‘Siemens SunField 6’ solar collector is of 95.2 m in length and of 5.776 m in width. The
reflective aperture area is 545 m2
and contains 8 modules which are organised row-wise along the
horizontal axis. Similarly, ‘SkyFuel SkyTrough’ solar collector is of 115 m in length and of 6 m in
width. The reflective aperture area is 656 m2
and contains 8 modules which are organised row-
wise along the horizontal axis. The DNI is an important factor to generate solar power successfully.
It measures the amount of solar energy falling per square metre per day at a certain location. This
PTC tracks the sun from the East horizon to the West horizon, and direct normal irradiance (DNI) is
simultaneously calculated (Boukelia and Mecibah 2015). Two types of receivers were used for both
synthetic oil and molten salt plant. For Oil plant, ‘Royal Tech CSP RTUVR 70M4’ was used as recei-
ver. For this receiver, the inner and outer diameters of absorber tube are 0.066 m and 0.07 m,
respectively. Also, the glass envelope inner and outer diameters are 0.1196 m and 0.125 m, respect-
ively. Inner surface roughness was considered 0.00002 for oil plant. For molten salt plant, we used
‘TRX70-125’ as receiver. The inner and outer diameters of absorber tube are 0.066 m and 0.07 m,
respectively. Also, the glass envelope inner and outer diameters are 0.119 and 0.125 m, respectively,
and the inner surface roughness is 0.000045.
Another parameter Solar multiple (SM) is defined as the ratio of the actual aperture area of the
mirrors to the reference mirror aperture area (Boukelia and Mecibah 2015) and can be written as (1)
SMdp =
Q̂thermal SF
Q̂thermal PB
(1)
Figure 1. The block diagram of the methodology steps in this study.
4 N. BHUIYAN ET AL.

The working process flow diagram of a parabolic trough collector (PTC) system is illustrated in
Figure 2. As seen in the figure, the heated HTF is carried to a stream regenerator, which contains
heat exchanger and superheater. Then the hot water is evaporated to produce super-heated steam
which passes through the turbine.
Significant assumptions and different values of the key parameters, listed in Table 2, are used in
the solar field, power cycle and thermal storage design. To obtain desired power during the evening,
rainy day, and dark times, and to increase the possible efficiency, the PTCSPs have been combined
with fuel backup system (FBS) and full load hours TES (Kariuki, MacHinda, and Chowdhury 2012;
Kordmahaleh et al. 2017; Cinocca et al. 2018) Considering their high cost and vapour pressure, Ther-
minol VP-1 cannot be used as a TES medium. In this study, the most popular two-tank molten salt
storage system is considered for TES. The molten salt does not react with others due to its low
pressure and low chemical reactivity. It has moderate heat storage capacity with required specific
heat value (Fernandes et al. 2012). Hence, this study uses direct TES for the plant with Hitec
solar salt and indirect TES with Theminol VP-1 as HTFs for the proposed PTCSP.
The full load hours of TES of the plant specifies the number of hours that can operate without
direct irradiance as given by (2) (Boukelia and Mecibah 2015);
Estorage =
wdctDn
hcycle
(2)
Table 1. Input Parameters for both synthetic oil and molten salt plant.
Input data parameters Oil plant Salt plant
Solar multiple 3.7 3.8
Field HTF Therminol VP-1 Hitec Solar Salt
Collectors (SCAs) Siemens SunField 6 SkyFuel SkyTrough (with 80-mm OD receiver)
Receivers (HCEs) Royal Tech CSP RTUVR 70M4 TRX70-125
Power cycle Rankine cycle Rankine cycle
Cooling method Hybrid cooling Hybrid cooling
Figure 2. Schematic diagram of parabolic trough solar thermal power plant.

Due to less contribution by the SF and TES, auxiliary energy is used to keep the design point at a
moderate level, as expressed by Larraín, Escobar, and Vergara (2010):
Q̂back = m̂(hin tur − hout SF) (3)
fback =
Q̂back
Q̂total
(4)
In summary, the energy analysis and economic viability of the optimised plants are modelled using
SAM environment (“System Advisor Model (SAM),” 2017). The results obtained are exported to
Origin 8.5 to make further calculations for 4E analysis.
2.2. Thermodynamic analysis
The numerical model for the thermodynamic analysis of an individual component of the
plant is presented in this section. In this model, we assume the steady-state operating con-
ditions with insignificant changes in kinetic and potential energy. The details are explained
as follows.
2.2.1. Energy analysis
Total direct normal irradiance collected by the solar field area is expressed as
Q̂incident = Ib · A · cos u (5)
Energy supplied by the solar field aperture area is expressed as follows:
Q̂useful = m̂f · (hSF0 − hSFi) (6)
Table 2. Key design parameters of two power cycles.
Parameters Therminol VP-1 Hitec Solar Salt
Solar Field Block
Design loop (inlet/outlet) temp (°C) 296/393 293/550
Field HTF operating (min/max) temp(°C) 12/400 238/593
Field flow (min/max) velocity (m/s) 0.357/4.77 0.269/2.15
Mass flow (min/max) rate (kg/s) 1/11.5 1.75/12.8
Single loop aperture (m²) 6540 7872
Actual number of loops 135 97
Field thermal output (MWt) 599.27 515.922
Row Spacing (m) 15 15
Solar irradiation at design (W/m²) 950 950
Loop optical efficiency 0.74 0.73
Length of single collector module (m) 11.9 14.375
Collector optical efficiency at design point 0.86 0.85
Heat loss at design (W/m) 175 166.25
Power Block
Rated cycle conversion efficiency 0.4 0.415
Design (inlet/outlet) temp (°C) 393/296 550/293
Aux heater outlet temp (°C) 393 550
Thermal Storage Block
Storage volume (m3
) 11135 4132.71
Tank diameter (m) 26.6248 16.2203
Min fluid volume (m3
) 556.752 206.636
Fluid Temperature (°C) 344.5 421.5
TES fluid density (kg/m3) 1870.9 1821.93
TES specific heat (kJ/kg-K) 1.50225 1.5155
6 N. BHUIYAN ET AL.

Therefore, solar field energy efficiency is given as:
hen SF =
Q̂useful
Q̂incident
(7)
The power block energy efficiency is given as:
hen PB =
PGnet
Q̂received
(8)
The plant’s final energy efficiency is calculated as follows:
hen overall =
PGnet
Q̂incident
(9)
The designed 50 MW power plant capacity factor is expressed as:
CF =
PGnet
Dn · 24
hr
day

· 50MW
(10)
2.2.2. Exergy analysis
The solar field received exergy is expressed as (Winter, Sizmann, and Vant-Hull 1991; Ravi Kumar
and Reddy 2012):
Êxrecived SF = Q̂useful · 1 −
4Tambient
3Tsun
(1 − 0.28lnf )

(11)
It depends on the dilution factor, f (1.3 × 105
), and is defined by mixing ratio of solar radiation
from the sun (Tsun) and radiation from the surroundings (Ravi Kumar and Reddy 2012).
The useful exergy supplied by the receiver is given by (Singh, Kaushik, and Misra 2000):
Êxuseful = m̂SF · (ÊxSF0 − ÊxSFi) = m̂f · [(hSF0 − hSFi) − Tambient(SSF outlet − SSF inlet)] (12)
Therefore, solar field exergy efficiency is given by:
hex SF =
Êxuseful
Êxrecived SF
(13)
The power block exergy efficiency is presented as:
hex PB =
PGnet
Êxrecived PB
(14)
So, the overall exergy efficiency of the plant is given as follows:
hex overall =
PGnet
Êxrecived SF
(15)
2.3. Environmental impacts analysis
Environmental impacts include natural gas preservation, life cycle gas emission, land use and water
consumption. Its study is necessary for harnessing energy from the CSP based plant, to sort out the
possible benefits and to identify the energy production stages to be developed (Lechón, de la Rúa, and
Sáez 2008). PTCSPs, a kind of renewable energy power plant, help to preserve fossil fuels and reduce

the CO2 emission. For producing 1 kWh electricity, a 28.32 m3
natural gas is required while a 0.62 kg
CO2 (0.35 m3
) is emitted (Brander et al. 2011). Table 3 illustrates the amount of fossil fuel preser-
vation and CO2 reduction data obtained from the lifecycle of the considered power plants.
As seen from the Table, the volume of fossil fuel conserved yearly is considerable. The amount of
natural gas saved due to the proposed PTCSP could be used for a higher conversion eﬃciency appli-
cation. Furthermore, the reduced CO2 emission is essential to safeguard the environment.
2.4. Economic analysis
The LCOE is considered as one of the key parameters for economic studies of a CSP plant (Dersch
et al. 2004). The economic study of the presented CSP plant has been performed in terms of total
capital costs and the LCOE. The values of these two parameters depend on the inputs and key
assumptions. The inputs of this study were taken from the available databases and preceding studies
(Blair et al. 2014). The economic calculation is done by using the SAM software. The LCOE is given
by (16):
LCOE =
FCR × TCC + FOC
AEP
+ VOC (16)
where
FCR = CRF × PFF × CFF (17)
and
CRF =
wacc
1 −
1
(1 + wacc)N
(18)
and
PFF =
1 − (TAX × PVDEP)
1 − TAX
(19)
and
CFF =

c
c=0
CONc × (1 + (1 − TAX) × ((1 + CINT)(c+0.5)
− 1)) (20)
2.5. DNI Estimation
Precise time series solar irradiance information at the area of interest is required to set-up any solar
energy based energy conversion system and validate the optimum model performance. Undeniably,
hourly solar irradiance data over Typical Meteorological Year (TMY) are needed for accurate calcu-
lation of the output energy gain (Pérez-Higueras et al. 2012). For a speciﬁc location, the TMY data
can be found by measuring data continuously over an extended period of time. The TMY infor-
mation of the considered sites, for instance, relative humidity, ambient pressure, dew point tempera-
ture, dry bulb temperature and wind speed are generated using ‘Meteonorm’ – a reliable data source
Table 3. Natural gas preservation and CO2 gas mitigation during the life cycle of the plant.
Parameters Salt plant Oil plant
Natural gas preservation (m3
) 4,844,968,381 4,810,833,039
CO2 mitigation (m3
) 59,877,787.20 59,455,916.80
8 N. BHUIYAN ET AL.

and sophisticated calculation tool (Mohammad and Mishra 2019). The solar irradiation data is
applied to a methodology, which is based on experimental models, because it is easy to give the
inputs and the computation cost is low (Yang, Koike, and Ye 2006). The model comprises of the
following sets of equations:
(1) Declination of the sun (d) and angles of the sunrise hour (vs) are estimated using the follow-
ing equations (Duffie and Beckman 1991):
d = 23.45 sin (Dn + 284) ·
360
365

(21)
vs = arc cos [−tan (d) tan (w) ] (22)
(2) Daily extra-terrestrial irradiance (Bo) and maximum possible sunshine duration (Sd max) are
calculated from the following equations (Duffie and Beckman 1991):
Bo =
24 × 3600 × S
p

· dr · cos (w) sin (vs) cos (d) +
pvs
180
sin (d) sin (w)

(23)
dr = 1 + 0.033 cos
360
365
J

(24)
Sd max =
2vs
15
(25)
(3) Daily global irradiance on horizontal surface (HG) is calculated by the equation (Mecibah et al.
2014):
HG = 0.57089 + 0.01028
Sd
Sd max

− 0.00005
Sd
Sd max
2

× Bo (26)
(4) Daily diffuse irradiance on a horizontal surface (HD) is calculated from the equation (Boukelia,
Mecibah, and Meriche 2014):
HD = 0.337 − 0.068
HG
H0

− 0.025
HG
H0
2
− 0.002
HG
H0
3

× HG (27)
(5) Hourly horizontal plane global irradiance (G) can be calculated by the equation (Collares-Per-
eira and Rabl 1979):
G = rt × HG (28)
rt =
p
24
· (a + b × cos v ) ·
cos v − cos vs
sin vs −
pvs
180

cos vs
⎛
⎜
⎝
⎞
⎟
⎠ (29)
where
a = 0.0490 − 0.5016 sin (vs + 1.047) (30)
b = 0.6690 + 0.4767 sin (vs + 1.047) (31)

(6) Hourly horizontal plane diﬀuse irradiance (D) is calculated using the equation (Liu and Jordan
1960):
D = rd × HD (32)
rd =
p
24
·
cos v − cos vs
sin vs −
pvs
180

cos vs
⎛
⎜
⎝
⎞
⎟
⎠ (33)
(7) Hourly horizontal plane direct irradiance B is given by the equation:
B = G − D (34)
(1) Hourly direct normal irradiance can be calculated by:
I =
B
d + cos d × cos v × cos w
(35)
3. Results and discussions
3.1. Performance optimisation of the plant
The objectives of the optimisation are to find the lowest LCOE and the highest annual energy gen-
eration. The main parameters of the optimisation process are full load hours of the TES and SM. The
meteorological data of several zones of Bangladesh are taken from the SAM software to simulate
annual performance. The Cox’s Bazar zone with the latitude of 21.38°N and longitude of 91.97°E
has been selected owing to its maximum direct normal irradiance (DNI), which is 4.12 kWh/m2
/
day. The configured PTCSP for the Cox’s Bazar is examined for several key parameters at a certain
DNI. The LCOE and annual energy generation per year have been calculated by varying the full load
hours of the TES and solar multiple. Variation of the capacity factor, annual energy, and LCOE for
different types of oil and salt with respect to the solar multiple is shown in Figure 3. As seen, the
Therminol VP-1 and Hitec solar salt have the maximum capacity factor and annual energy, and
minimum LCOE. For this reason, therminol VP-1 is taken for thermal oil plant optimisation, and
Hitec solar salt is taken for salt plant optimisation. Both capacity factor and annual energy increase
linearly with the solar multiple for both salt plant and thermal oil plant which are shown in Figures 4
and 5, respectively.
Sizing of the solar field aperture area has a direct effect on the LCOE and annual power gener-
ation. If we increase the solar field aperture area, system’s electric output increases, thereby reducing
the LCOE.
An optimal solar field area is used to maximise thermal energy generation. It minimises the instal-
lation and operating costs, and yields a cost-effective TES. The SM versus annual energy generation
and LCOE for thermal oil plant and molten salt plant are presented in Figure 6(a,b), respectively. Up
to a 3.7 solar multiple, the LCOE rapidly decreases and then it gradually increases for both the syn-
thetic thermal oil plant and molten salt plant. This is due to the capital cost increment. Therefore,
with increasing solar field area, a significant portion of energy goes unused. For the higher energy
production, the average value of the LCOE is minimal, determined to be 10.05 and 9.86 ¢/kWh
for the thermal oil plant and the salt plant, respectively. All economic data for both types of plants
are summarised in Table 4.
The LCOE and annual energy generation over the full load hours of TES are shown in Figure 6.
When the energy from the sun is not available, thermal storage backup system can be used for higher
annual energy generation. This increases system efficiency and potentiality for longer period of
10 N. BHUIYAN ET AL.

operation. From Figure 6(a,b), it can be seen that with the increment of full load hours of TES, the
LCOE gradually decreases to a lowest value of 10.05 ¢/kWh for the thermal oil plant and 9.86 ¢/kWh
for the salt plant. Further increase in full load hours of TES cause the LCOE to decrease for both
types of plants. Because of higher capital cost and mostly for short duration design capacity, huge
potential of the full load TES system still remains untapped.
Figure 3. Eﬀectiveness of various HTF (a) In terms of capacity factor, (b) In terms of annual energy and (c) In terms of LCOE.
Figure 4. Eﬀectiveness of thermal oil plant (a) In terms of capacity factor and (b) In terms of annual energy.

A signiﬁcant portion of the TES potential is still in misuse, which makes the plant less economical
than as expected. In molten salt technology, direct storage mode is used. On the other hand, in ther-
mal oil plant technology, indirect storage mode is used. In contrast with indirect storage mode, mode
of direct storage is more commercially feasible. However, some of the major disadvantages of the
indirect storage mode are as follows: (1) it requires a heat exchanger, (2) large amount of storage
medium is required, and (3) capital cost is higher.
Figure 5. Eﬀectiveness of salt plant (a) In terms of capacity factor and (b) In terms of annual energy
Figure 6. The variation of annual energy generation and LCOE with solar multiple for (a) Thermal oil plant and (b) Salt plant.
Table 4. All economic data for both types of plant.
Economic Data Parameters Oil plant Salt plant
Financing cost ($) 18,465,621 18,190,745
Levelized cost of generation (LCOG) (cents/kWh) 11.78 11.54
LCOG O and M (cents/kWh) 2.38 2.36
LCOG depreciation (cents/kWh) 6.75 6.58
Nominal LCOE (cents/kWh) 10.05 9.86
Net capital cost ($) 184,805,072 181,594,528
Net capital cost per watt ($/W) 3.73 3.67
Present value of OM ($) 51,646,921 51,710,000
Present value of annual costs ($) 180,140,033 177,962,000
Present value of fuel OM ($) 0 0
Present value of non-fuel OM ($) 51,646,921 51,710,000
Weighted average cost of capital (WACC) ($) 8.53 8.52
Total installed cost ($) 166,339,221 163,404,112

Table 5. 4E comparative analyses of the studied plants.
Parameters Andasol 1
Oil plant (in this
study)
Oil-andasol 1 diﬀ
(%)
Oil plant (Boukelia et al.
2015)
Salt plant (in this
study)
Solt-andasol 1 diﬀ
(%)
Salt plant (Boukelia et al.
2015)
Annual energy (GWh) 179.1 169.8 -5.47 225.98 171.1 −4.67 236.90
Aperture area (m²) 510,120 745,560 31.53 541,786 763,584.00 33.19 570,004
Storage volume (m³) 16108.7 11135 −44.84 11121.90 11327.1 −42.21 7263.90
Capacity factor (%) 41.5 39.2 −5.86 50.50 39.5 −5.06 54.60
Annual water usage (m³) 844722 49,312 −71.3 822,466 47,061 −79.49 800,482
Land use (acres) 529 670 21.04 562 660 19.84 592
Total capital cost ($) 411,690,000 184,805,072 77.72 398,949,722 181,594,528 77.32 359,052,834
Nominal LCOE (cents/
KWh)
10.13 10.05 −0.79 10.03 9.86 −2.7 8.48
Storage capacity (Hour) 7.5 8 6.25 5.5 7 −7.14 10.5
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3.2. Optimised plants comparison
A comparative study, based on 4E analysis, of the two optimised plants with a reference plant is pre-
sented in Table 5. The reference plant ‘Andasol 1’ is located in Guadix, Spain. In that location, the
average DNI is about 2136 kWh/m2
/year. The optimised plant in this study is located in Cox’s-Bazar
(Bangladesh) where the average DNI is only 1503 kWh/m2
/year. The capacity of the both optimised
plants is 50 MW, which is equal to the capacity of the reference plant.
Though reference plant has the same capacity of 50 MW, yet the design and ambient con-
dition are different. Thereby, some of the design parameters as well as the results are different.
The reference thermal oil plant uses synthetic oil as the HTF, whose temperature range is 296–
393°C. In the molten salt plant, the temperature range is 286–550°C, which is higher than that of
thermal oil plant. The plants in this study have a considerable variation of aperture areas because
of the dependency on the solar multiple to fulfil the demand of 50 MW capacity. The thermal oil
plant and the salt plant have 31.53% and 33.19%, respectively, greater aperture area than that of
the reference plant.
Figure 8. Annual average direct normal irradiance for different places in Bangladesh.
Figure 7. The variation of annual energy generation and LCOE with full load hours of TES for (a) Thermal oil plant and (b) Salt plant.

Table 6. The viability analysis of the optimised plant for condition of Bangladesh.
Parameters
Cox’s Bazar (21.43 °
N,91.97°E)
Chittagong (22.27°N,
91.82 °E)
Dhaka (23.77°N,
90.38 °E)
Pabna (24.13°N,
89.05 °E)
Khulna (23.18°N,
89.17 °E)
Rangpur (25.73°N,
89.23 °E)
Sylhet (24.9°N,
91.88 °E)
Rajshahi (24.85°N,
89.37 °E)
Gross-to-net conversion
(%)
91.9 91.1 89.6 89.9 88.5 89 89 90.1
Capacity factor (%) 39.5 38.5 32.6 32.4 30.3 30.1 27.2 32.4
Annual energy
generation (GWh)
171.08 166.75 141.41 140.65 131.44 130.54 117.98 140.28
Annual water usage
(m3
)
47,061 46,855 45,148 45,003 44,503 44,278 43,275 44,974
Nominal LCOE (cents/
KWh)
9.86 10.1 10.82 11.88 12.68 12.76 14.07 11.91
Natural gas uses
(Millions-m3
)
4,845 4722 4005 3983 3722 3697 3341 3973
CO2 emission (Millions-
m3
)
59.88 56.36 49.49 49.23 44.00 45.69 41.29 49.10
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Thermal oil plant and salt plant storage volumes are found to be 44.84% and 42.21%, respectively,
which are lower than the reference plant. Indirect storage mode is used in the reference plant and in
the thermal oil plant while direct storage mode is used in the salt plant. The storage volume is differ-
ent as a result of full load hours and storage mode. The salt plant has a highest potential storage sys-
tem of about 11 h of operation due to high dispatch of the TES.
The primary challenge of using solar salt as the HTF is its having a comparatively high freeze-protec-
tion temperature (220 °C). Freezing of solar salt has been avoided as it can block the HTF and can rupture
the tube. The reference plant has a higher exergy efficiency compared to the plants studied. The optim-
isation is considered only in terms of annual energy and the LCOE. The reference plant has a higher
capacity factor of 41.5% and higher annual energy generation of 179.1 GWh shown in Figure 7(a,b).
In terms of storage capacity, the molten salt plant is optimised at 7 h storage system while thermal
oil plant and the reference plant has 8 and 7.5 h, respectively. Molten salt uses 79.49% less water than
that used in the reference plant. Thermal oil plant produces less CO2 gas emission as it produces
lower annual energy generation compared to the molten salt and the reference plant. The proposed
oil plant and salt plant require more land of 670 acres and 660 acres, respectively, compared to the
reference plant, which requires only 529 acres.
In terms of economic consideration, the salt plant technology would be the best choice compared to
the thermal oil plant, as the former has a lower investment cost, lower LCOE and higher capacity factor.
3.3. Feasibility study of the optimised plant
For reliable operation of the CSP plant, the average requirement of DNI must be more than
2000 kWh/m² (Noor and Muneer 2009). Here, from the Figure 8, it can be seen that the DNI varies
from 4 to 6.5 kWh/m² all over the country. Considering some divisional region as shown in Figure 8
the highest DNI is obtained for the Cox’s Bazar region.
The feasibility study requires due consideration of the factors such as environmental resources,
land, surroundings, infrastructure interconnections, hybridisation with other fuels, water availability,
natural hazards risks, labour cost, permissions of the authority and so forth. The parabolic trough
solar plant based on molten salt in Bangladesh is feasible for the following reasons: (i) more than
2000 kWh/m² average DNI, (ii) moderate flat land area, (iii) nearby grid-connected transmission
network, (iv) availability of water resource, (v) hybridisation with other fuel, and (vi) lowest possi-
bility of natural hazards.
The feasibility investigation with consideration of 4E for the optimised salt plant has been carried
out in eight different regions in Bangladesh where the requirement stated above are satisfied.
Table 6 shows that the lowest LCOE is 9.86 cents/kWh which is in Cox’s Bazar and the highest
LCOE is 14.07 cents/kWh in Sylhet region. In regard to the capacity factor, it is noted that the
capacity factor is higher in Cox’s Bazar region compared to the Sylhet region. These values are
39.5% and 27.2%, respectively, for Cox’s Bazar and Sylhet region. The maximum annual power gen-
eration is 171.08 GW in Cox’s Bazar and the minimum power generation is 117.98 GW in Sylhet.
Water consumption is ranged from a minimum value of 43275 m3
for Cox’s Bazar to a maximum
of 47061 m3
for the Sylhet region. The CO2 emission in Cox’s Bazar region is 59.88 Millions-m³
whereas this is only 41.30 Millions-m3
in Sylhet.
In summary, this study presents a thorough economic analysis based on LCOE. It can be seen that
among all the parameters, the LCOE is the most influential. It depends mainly on annual energy gen-
eration because the investment costs for similar size plants are similar. Considering all parameters,
Cox’s Bazar region is the best suited for the large-scale CSP plant.
4. Conclusions
In this work, a 50 MW PTCSP is designed by utilising two types of HTF, (i) Therminol VP-1, and (ii)
solar salt with full load hours of TES back-up. The optimisation procedure is based on a parametric

study of the full load hours of TES and solar multiple by taking the Andasol 1 as a reference plant.
The reference plant has slightly higher annual energy and exergy. However, the salt plant has lower
total annual costs than that of the reference plant. Water requirement in the molten plant is lower
(47061 m³) than that of the reference plant (84472 m³). The reference plant requires less land area
than that of the optimised salt plant. In terms of economic consideration, molten salt technology is
found as the better option compared to other plants because of its lower LCOE (9.86 ¢/kWh) and
investment cost. The viability study of the optimised molten salt PTCSP in eight different
regions in Bangladesh is conducted. The results show that the Cox’s Bazar region is the best location
for the PTCSP due to its lower LCOE of 9.86 ¢/kWh and higher annual power generation of
171.1 GW.
Based on this study, it is recommended that by utilising PTCSP, the present power crisis in
Bangladesh can be substantially lessened. The government should take necessary steps and adopt
strategies to generate power from this emerging parabolic trough solar power plant technology.
ORCID
Nur Mohammad http://orcid.org/0000-0001-5872-2856
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