Global Journal of Engineering Sciences (GJES)
Investigating
an Integrated Solar Combined Cycle Power Plant
Authored by A Rashad
Abstract
Using
solar energy standalone to generate electricity has high investment risk. This
is due to the need to energy storage systems to ensure electricity generation
during the night. For this reason the hybridization of renewable energy
resources and fossil fuel has been motivated. In an Integrated solar
combined-cycle (ISCC) the solar thermal energy is integrated into combined
cycle gas turbine (CCGT) power plant. The aim of this study is to evaluate the
impact of addition of solar energy to a CCGT at both design and off design
conditions of solar thermal input and ambient temperature.
The
evaluation was fulfilled by studying the main performance indicators for
hybridization of combined cycle (CC) with solar energy. These factors are the
solar conversion efficiency, solar fraction, and boosting factor.
The
study was implemented on Kurymat ISCC, in Egypt. The plant is designed to
produce 135 MWe. It composed of parabolic trough solar field integrated with a
conventional CCGT power Plant. The design solar heat input is 50 MWth at 20 °C
dry bulb ambient temperature. The CCGT consists of one gas turbines of 70 MWe,
one HRSG that produce steam at pressure 90 bar and a steam turbine of 65 MWe.
The study shows that, for night mode operation (no solar) changing the ambient
temperature from 5oC to 35 °C, the plant efficiency drops from 0.53 to 0.51 and
the output power changes from 119.2 MW to 99.69 MW. Also, for day mode (with
solar) at design solar thermal input and ambient temperature the ISCC
efficiency is higher than CC efficiency when we neglect the solar fuel cost,
while the efficiency drops below that of the CC if the solar fuel cost is
considered. The power output reduces with increasing the ambient temperature
and increases with increasing the solar thermal input.
Keywords:Parabolic trough; Integrated solar
combined cycle; Thermodynamic analysis
Abbreviations
CCGT -
combined cycle gas turbine
CST -
Concentrated solar thermal
DSG -
direct steam generation
HPEC -
High pressure economizer
HPEV -
High pressure evaporator
HPSH -
High pressure super heater
HRSG -
Heat recovery steam generator
HTF -
Heat transfer fluid
ISCC -
Integrated solar combined cycle
LCOE -
Localized cost of energy
LEC –
Levelized energy cost
MENA -
Middle East North Africa
NGCC -
Natural gas combined cycle
SAPG -
Solar aided power generation
SSG -
Solar steam generator
Nomenclature
dQsolar Increase
of solar heat input MW
dηsteam Increase
of steam cycle efficiency -
Fsolar Solar
fraction -
LHVgas Lower
heating value of natural gas MJ/kg
Qsolar Solar
heat input MW
Wfossil Output
power of the NGCC MW
Wfossil+solar Output
power of the ISCC MW
Wnet The
net power of the whole plant MW
Wsolar The
output power of the steam turbine with solar hybridization MW
ηplant Overall
plant efficiency %
ηsolar Solar
conversion efficiency %
Θ s Boosting
factor of the steam cycle efficiency
Introduction
Electricity
generation from solar energy has been considered a feasible alternative for
fossil thermal plant due to the fear of fossil fuel depletion. By using solar
energy standalone to generate electricity, energy storage becomes a must to
ensure electricity generation during the night. The systems of energy storage
are characterized by high cost that causes a dramatic increase of the price of
the generated electricity i.e, high investment risk [1]. For these reasons, the
reliability of solar thermal power plant as a base load power source is not
high. This problem caused searching new methods to reduce the power plant
capital cost.
The
willing of providing cost-effective improved and stiff electricity supply
motivated the hybridization of renewable energy resources and fossil fuel in
electricity production [2]. Also, the hybridization of fossil fuel and
renewable energy is capable to achieve substantial economic and environmental
benefits [3-6]. One method of hybridization of fossil fuel and renewable energy
is to use a solar integration with fossil fuel power plant. This way fulfills
effective use of solar and fossil resources, overcoming the problem of sunlight
discontinuity. Moreover, it reduces the release of greenhouse gas. Also, the
elimination of the thermal storage system reduces the plant cost [7-10].
Elmohlawy
et al [2], Abdel Dayem et al [5] studied the impact of integrating solar energy
into CC on the CO2 emission. Elmohlawyet al [2] developed a Mathcad
mathematical model to simulate a proposed ISCC power plant with parabolic
trough collectors under Egypt climate condition. In the first configuration the
solar steam generator (SSG) is used to evaporate and superheat part of the feed
water of the intermediate pressure economizer. In the second configuration the
SSG is used to superheat the feed water extracted from the deaerator. The
results revealed that the second configuration results in more increase in net
output power than the first one. Also, the second configuration generates more
solar electricity than the first one. The solar integration reduced the CO2
emission by 51670.82 ton/year. Abdel Dayem [5] simulated a proposed integrated solar
combined cycle power plant under north Benghazy/ Lybia climate conditions. The
study was implemented on two modes of operation: first; fuel saving mode where
the solar steam is utilized to preheat the air before entering the gas turbine
combustion chamber, second; power boosting mode where the solar steam is
injected in the steam turbine. The results illustrated that the fuel saving
mode reduced the CO2 emission by 7972.25 ton/year and the benefit/cost ratio is
1.74/year.
Nathan
et al [3], and Bahroon et al [6] examined the economic impact of solar energy
integration into combined cycle. Nathan et al [3] made a review in order to:
first; pinpoint the gain of concentrated solar thermal (CST) energy
hybridization and combustion technologies. Second; pinpoint and categorize how
CST and combustion can be merged in a hybrid system. Third; pinpoint the
promising hybridization technologies for carbon-neutral or carbon-negative
emission. Finally; pinpoint the challenges that face the development of such
technologies. They observed that there are two classes of concentrated solar
thermal hybrids. The first one is employed to preheat the feedwater to a
regenerative Rankine cycle which is named Solar Aided Power Generation (SAPG).
This category has 3-5% annual solar share and it is limited to certain sites.
The second one is hybrid solar receiver-combustor. By this technology the
Localized Cost of Energy (LCOE) can be reduced up to 17% and get a saving in
fuel consumption to 40%. Bahroon et al [6] developed a model to predict the
beam radiation, the annual fuel consumption, the fuel cost, and the LCOE for an
ISCC in Yemen working with coal and heavy oil. It was found that that the LCOE
was higher when using heavy fuel oil than in case of coal.
AE
Elmohlawy, et al. [7] elaborated two thermodynamic models to assess the
performance of a two configurations of integrated solar combined cycle power
plants with parabolic trough solar collector. In one configuration, the steam
generated by the solar collector is injected into the high-pressure section
while in the second configuration it was injected into the intermediate
pressure section. The plants were proposed to operate under climate of southern
Egypt. The results showed that the first configuration experienced higher increase
in the output electrical power, net thermal efficiency, and solar to electric
conversion efficiency than the second configuration.
A
Rovira et al. [8] proposed a new configuration of ISCC includes a gas turbine
with partial recuperation. The generated solar steam is injected into the
high-pressure section of the evaporator. Partial recuperation reduces fuel
consumption without changing the gas turbine operating conditions and hence,
without decreasing its efficiency. The proposed new configuration of the ISCC
was investigated in detail with two case studies: Las Vegas and Almeria; have
been compared. Improvement of the minimum yearly heat rate of 1% and 1.8% in
Almeria and Las Vegas respectively was observed. Concerning the cost analysis,
the ISCC with partial recuperation achieved a lower LCOE by 0.9% and 1.1% for
Almeria and Las Vegas respectively.
AE
Elmohlawy, et al. [11] developed a model in Mathcad to investigate the
performance of two layouts of ISCC. In both layouts the reference combined
cycle is a triple pressure conventional gas turbine cycle. In the first layout,
a fraction of feedwater is extracted from the intermediate pressure economizer
and is superheated in a solar steam generator, then is injected into the exit
from the intermediate pressure turbine. In the second layout, portion of
feedwater is taken out the deaerator where it is evaporated in a solar steam
generator and then is injected into the high-pressure steam heater. The
analysis was implemented for climate conditions for a site in Aswan city,
Egypt. The results showed that the first layout had a solar to thermal
electricity efficiency of 37.6 % in summer and 11 % in winter. Also, it has a
43 MW increase in output power in ummer and 12 MW in winter. On the other hand,
the first layout has 37 MW increase in output power in summer and 10 MW
increase in winter.
AO
Binamer [12] developed a mathematical model to assess the performance of ISCC
power plant. The plant was planned to be erected in Kuwait with a 60 MWe
capacity parabolic trough solar collector. Engineering Equation Solver (EES)
was used to develop the model. The model was used in a sensitivity analysis to
study the effect of certain parameters such as solar heat input. The results
showed that the plant efficiency could increase up to 100% more than the
conventional one. Also, the output power is strongly affected by solar heat
input. It is more beneficial to add thermal energy storage than increase the
solar fraction from 0.2 to 0.3. by integrating thermal solar the annual
emission was cut down and the Annual fuel saving was grown. These benefits
encourage the implementation of this type of plants in Kuwait.
Achour
et al [13], Manente[14], A Boretti et al. [15], Y Liang et al. [16] studied the
influence of integrating solar energy into combined cycle on the efficiency.
Achour et al [13] developed a thermodynamic model to estimate the thermal
performance of an ISCC in Algeria. The solar to electricity efficiency was
found to be up to14.4% while the overall plant efficiency was up to 60%.
Manente [14] developed a model to assess the integration of solar energy into
CC with including new equipment or without changing the existing equipment. The
results illustrated that the solar to electricity efficiency was found to be 24
to 29%. Moreover, the thermal efficiency of the integrated cycle was lowered
due to the drop-in gas turbine efficiency at reduced loads. A Boretti et al.
[15] Developed a computational analysis for a simulated ISCC to be located in
Trindad and Tobago. The proposed plant is supposed to have a parabolic trough
as a solar collector without thermal energy storage. The purpose was to
identify the benefits gained in fuel conversion efficiency. The results showed
that an increase of about 2.58 % in the fuel conversion efficiency could be
obtained for 8 hours operating period of the solar field per day. The increase
was about 3.16% for 4 hours operating period per day. Y Liang et al. [16]
proposed an ISCC consists of super critical CO2-Brayton cycle and organic
Rankine cycle as a CGS. An optimization study was implemented using a nonlinear
programming mode. For this ISCC the results showed that, an increase in thermal
efficiency up to 3.6% could be obtained.
Brodrick
et al [17], Manente et al [18] studied the optimization of integration of solar
energy into combined cycle. Brodrick et al [17] investigated many designs of
ISCC to get better configuration with annual solar contribution than the
published designs. In this optimization analysis a bi-objective Pareto Fronts
was constructed. Manente et al [18] used model built by Thermoflex to
investigate three technologies of solar integration: parabolic trough, Linear
Fresnel, and solar tower for optimum integration. The results revealed that the
best performance was achieved by utilizing moderate temperature concentrating
solar collectors due to reduced heat transfer irreversibilities. In turn the
solar radiation-to-electricity conversion efficiency was up to 30%.
Previous
literatures give valuable information into the advantages of the ISCC
technology and the best configurations, but examination of solar integration
into CC power plant based on data of real working plant is rare. The main
objective of the current work is to investigate in detail the effect of solar
integration into an existing CCGT. The goal is to obtain the best integration
conditions of the thermal energy into the CCGT cycle, thus obtaining the best
plant performance. This goal was achieved by a detailed thermodynamic analysis
of the Kurymat ISCC. The physical plant description, various parameters and
design data are utilized to study the effect of solar hybridization into
combined cycle under design and off-design operations for a range of
metrological input parameters such as ambient temperature and solar thermal
input. This work is organized as follows: The Kurymat ISCC Power Plant is described
in detail in section 2. The design data and various parameters of the plant at
design and off-design conditions are shown. Performance evaluation for the
plant in both day mode and night mode operation at design point and at
off-design conditions is presented in section 3. Finally, the main performance
indicators for hybridization of CC with solar energy which are the solar
conversion efficiency, solar fraction, and boosting factor are investigated at
different metrological conditions in section 4. A proposal for the calculation
of the ISCC efficiency was also introduced.
The Kurymat ISCC Power Plant
Kurymat
ISCC [19] is the first plant of this type erected in Kurymat (29°.27` N), Egypt
and has been operated interruptedly since July 2011. The plant is one of the
first three of its kind in the Middle East North Africa (MENA). The other
plants are located in Algeria and Morocco. Figure 1 illustrates the schematic
diagram of the Kurymat ISCC power plant.
The
plant consists of two main subsystems; the solar field subsystem and the
combined cycle subsystem. The solar island consists of parallel rows of single
axis tracking trough. The trough axes are oriented to north-south and track the
sun as it moves in the sky from east to west. In reference day mode situation
of 700 W/m2 direct normal irradiation at solar noon of 21 March and 20 °C
ambient temperature, the solar field has a thermal capacity of 50 MWth; this
enables the ISCC to generate 135 MWe of gross electric power output. Without
solar heat, the plant generates 115 MWe electric power output. The difference
between the two modes of operation is 20 MWe. The solar field includes 40 loops
and each loop has four SKAL-ET 150 parabolic trough collectors covering 130,800
m2. The HTF is Therminol VP-1from Solutia Inc. operates between 12° to 400 °C
with mass flow of 250 kg/s at 100% load [20]. Hot HTF returning from the solar
field at 393 °C at a pressure of 20 bars is pumped through the solar heat
exchanger. The HTF leaves the solar heat exchanger at 293 °C and is pumped back
into the solar field. The design HTF temperature rise across the solar field is
on the order of 100 °C.
The
Combined Cycle is located outside of the power block in the North of the plant.
It has one heavy-duty gas turbine GE type MS6001FA with a 70 MWe at 20 °C
ambient temperature and one HRSG. The flue gas flow rate to the HRSG is about
206 kg/s from the gas turbine at temperatures of about 600 °C at full load
operation. The flue gas leaves the HRSG at about 100°C. The steam turbine is of
type Siemens SST 900 series single casing, horizontally split condensing type
steam turbine with a generator. At rated conditions of the gas turbine and HRSG
full load operation, solar heat input of 50 MWth and 20 °C ambient dry bulb
temperature the steam turbine generator output is about 65 MW. The design
parameters of the Kurymat ISCC and its major components are summarized
show the schematic and
T-s diagrams with state points at design conditions. In Figure 2, the
extraction and the injection points of steam in HRSG is shown. The steam is
extracted from the high-pressure economizer (HP EC2) to the solar field heat
exchanger and is injected into the high superheater (HP SH1). The values and
conditions at different state points in Figure 2 at design situation are
tabulated in Table 2. The various parameters and data of the plant at design
and off-design conditions are selected from manufacturer data base [21].
Plant Performance Analysisn
Thermodynamic analysis of the
Kurymat ISCC is presented in this section. The detailed physical plant
description and various parameters and data of the plant mentioned above are
utilized to assess the effect of solar hybridization into combined cycle under
design and off-design operations for a range of metrological input parameters;
for instance; ambient temperature and solar thermal input. This section
includes: first, performance evaluation for the plant in both day mode and
night mode operation at design point. Then, the performance evaluation is
carried out at off-design condition
Plant performance at design
condition
Figure 4 and Figure 5 show the
variation of plant efficiency and power output as a function of the ambient
temperature for night mode (no solar input). Figure 4 depicts the dependence of
the gas turbine efficiency on the ambient temperature. It shows that, when
ambient temperature changes from 5 °C to 35 °C the gas turbine efficiency drops
from 0.361 to 0.337 and the combined cycle efficiency drops from 0.532 to 0.51.
In Figure 5, the gas turbine power output decreased from 81 MW to 67 which is
about 17%.at the same temperature variation. The reduction in the output power
when rising the ambient temperature is owing to the reduction in the air mass
flow rate delivered into the compressor. Also, the combined cycle output power
is reduced from 119.2 MW at 5 °C to 99.69 MW at 35 °C (about 16 %).
Figure 6 and Figure 7 show the
variation of plant efficiency and power output for day mode at design solar
thermal input 50 MWth. Figure 6 illustrates that the thermal efficiency of the
ISCC increases over the combined cycle efficiency using the equation (5), while
it is decreased lower than the combined cycle efficiency using equation (6). At
the design ambient temperature 20 °C, the overall plant efficiency is reduced
from 0.529 to 0.511 at about 3.4 %.
Figure 7 illustrates
the change of the output power along with the ambient temperature. It is clear
that as a general trend, the output power decreases with increasing the ambient
temperature. Also, the figure shows that at 20 °C ambient temperature the ISCC
power output is 134.3 MWe, while the CC power output is 112.45 MWe (about 19.4
% increase).
Plant performance at off-design conditions
From subsection 3.1 at design
point where the HRSG receives solar heat input of 50 MWth and the ambient
temperature is 20 °C, the gas turbine output power is 70MWe and the steam
turbine output is about 65MWe, while the thermal efficiency is 0.529 for night
mode and 0.511 for day mode. In this subsection, the investigation for the
off-design conditions is carried out for different solar heat input varied from
0, 25, 50, 75, 100 MWth respectively at different ambient temperature varied
from 0, 5, 20, 35 °C. The state point’s conditions and values are obtained from
manufacturer data base as mentioned above in section2. Appendix A shows as an
example for state points data of ISCC in Kurymat at solar thermal input 100
MWth and 20 °C ambient temperature. In this analysis, data from twenty
combinations between solar thermal input and ambient temperature were used.
Figure 8 and Figure 9
illustrate the change of the overall plant efficiency with thermal heat input
at different ambient temperature. In Figure 8, the efficiency is based on
fossil fuel consumption only. It increases with increasing the solar heat
input. The solar heat here is regarded as a complimentary heat source. Most of
the literatures do that, i.e. they consider the solar heat input as a free
energy source, but this consideration may cause ambiguous conclusion. The solar
heat input is not free energy; it costs a lot to be collected, so it must be
taken into consideration when calculating the overall plant efficiency. In
Figure 9, when the overall plant efficiency is evaluated based on fossil fuel
consumption and the solar heat gain, it was shown that the overall plant
efficiency decreases with increasing the solar heat input. For example, at
design ambient temperature 20 °C, the overall plant efficiency decreased from
0.52 at 25 MW thermal heat gain to 0.485 at 100 MW thermal heat gain. Both
Figure 8 and Figure 9 show that the overall plant efficiency decreases with
increasing the ambient temperature. Figure 10 shows the change of the overall
plant power output with solar thermal heat input at different ambient
temperature. It is clear that the power output reduces with increasing the
ambient temperature and increases with the solar thermal input.
In this section,
parametric study for different solar thermal input at different ambient
temperature is presented to study their effects on the injection point
conditions and the performance indicators.
In Figure 11, the variation of
steam pressure at injection point with heat input at different ambient temperature
is illustrated. The steam pressure increases as the solar heat input increases,
thus, the pressure of water/steam in all heat exchangers increases. This means
the injection pressure and temperature of the HP evaporator (HP EV) increase.
Figure 12 shows the relation
between steam temperature at injection point and heat input at different
ambient temperature. The figure shows that the injection temperature increases
from about 279 °C at 25 MW to 315 °C at 100 MW solar heat input. It is noted
that, the ambient temperature has no effect on the injection point conditions.
Figure 13 illustrates the
change of the solar conversion efficiency and the steam cycle efficiency with
the solar heat input. From the figure it is obvious that the solar conversion
efficiency is higher than the steam cycle efficiency. The steam cycle
efficiency increases with increasing the solar heat input and it has asymptotic
manner. This is due to the increase in steam turbine output. Also, the figure
reveals that the solar conversion efficiency reduces with increasing the solar
heat input. Moreover, it is shown that the steam cycle efficiency decreases
with increasing the ambient temperature, while the solar conversion efficiency
increases with increasing the ambient temperature.
Figure 14 shows the variation
of the steam cycle efficiency boosting factor with the solar heat input. Also,
the figure relates the solar conversion efficiency and the boosting factor. It
is obvious from the plot that both of them have the same trend which means that
the solar conversion efficiency is driven by the boosting fact. Both of the
solar conversion efficiency and the boost factor increase with increasing the
ambient temperature and decreases with solar heat input. Figure 15 shows that
the solar fraction increases with increasing the ambient temperature solar heat
input.
Conclusion
In this work the response to
various amounts of solar hybridization of Kurymat ISCC in Egypt was
investigated from thermodynamic point of view. The investigation includes the
evaluation of the off-design performance. It is carried out at different
ambient temperatures and different solar heat input. The following can be
concluded from this study:
The plant has
solar conversion efficiency higher than steam cycle efficiency
• Utilizing solar
power to substitute fractional heat of the HP EV in the HRSG may offer a
considerable solar fraction up to about 26% which leads to saving of
fossil-fuel and a consequent decrease in greenhouse-gas emissions.
• High pressure inlet
steam temperature decrease with increasing solar heat input/p>
• High pressure inlet
steam pressure increase with increasing solar heat input
• Solar injection
point temperature increase with increasing solar heat input
• Solar injection
point pressure increases with increasing solar heat input
• Solar conversion
efficiency decrease with increasing solar heat input
• Steam cycle
efficiency increase with increasing solar heat input
• Boosting factor of
the steam cycle efficiency decrease with increasing solar heat input
• Solar conversion
efficiency decreases with increasing solar heat input
• Overall plant power
output increase with increasing solar heat input
• Solar fraction
increase with increasing solar heat input
• The effect of the
ambient temperature is:
Increasing the ambient
temperature leads to decrease of gas turbine efficiency, gas turbine power
output, solar injection point pressure, steam cycle efficiency, overall plant
power output, and the solar fraction
Increasing the ambient
temperature leads to increase of high-pressure turbine inlet steam temperature,
solar conversion efficiency, and the solar fraction.
• The overall plant
efficiency is an important factor. In many literatures it was calculated after
hybridization without taking into consideration the thermal heat input. It was
claimed that it is a free energy [9]. Thus, the thermal efficiency is found to
increase, but really the evaluation of the overall plant efficiency by this way
is not right, because the solar thermal input is not free energy. Actually, it
costs a lot to acquire that energy, so it is thought that solar thermal input
must be taken into consideration when calculating the overall plant efficiency.
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