INTRODUCTION
The global energy consumption has increased rapidly leading to the
global warming as a result of emissions of carbon dioxide
(CO2) and methane into the atmosphere [1] added to
the shrinkage in the fossil energy availability. All these made the
recourse for an efficient energy conversion and the development of
renewable energy ever more critical. The integration of solar energy via
the combination of concentrated solar power technology (CSP) with
Combined Cycle (CC) has resulted in less capital cost and continuous
power supply, in addition to thermal efficiency improvement and
CO2 emission reduction [2]. A number of CSP plants
are under development over the world. Typically there are nine
large-scale solar power plants of 354 MW installed in the Mojave Desert
and several others operating in Italy, Iran and North-Africa. One of the
systems that will be studied herein is that of Integrated Solar Combined
Cycle (ISCC) system incorporating the technology of Parabolic Trough
Collectors (PTC), which is the most mature technology among CSPs.
Moreover, The ISCC system is one way to deploy CSP with low investment
risk while retaining power dispatchability, and subsequently the Thermal
Energy Storage (TES) may be eliminated or significantly reduced for a
solar hybrid plant [3]. Besides, the integration of TES into ISCC
has the advantage to increase the solar share but may induce more cost
[4]. In the concept of ISCC the solar heat is introduced in the
bottoming Rankine cycle, hence a Heat Solar Steam Generator (HSSG) is
added which offers several advantages over the solar-only electric
generation system or Rankine cycle [5]. Okoroigwe et al. [6]
reviewed CCs coupled with SPT and compared to those using Parabolic
Trough Collector (PTC) technology and concluded that the former is still
immature. Zhu et al. [3] examined the thermodynamic impact of
integrating solar energy into CC plants and concluded that with a
thermal solar input of 200 MW into HRSG the output can be boosted from
475 MWe to about 558 MWe, which has a benefit on fuel saving and
pollution reduction. With PTC technology the solar energy is transferred
to synthetic oil in the absorber tube and then via an intermediate
oil-to-water/steam heat exchanger is transmitted into the Rankine cyle,
hence resulting in the so-called Heat Transfer Fluid (HTF) technology.
Another configuration is the Direct Steam Generation (DSG) used to
increase the power output during the sunny periods. Nezammahalleh et al.
[7] considered three configurations of ISCC-DSG technology, ISCC-HTF
technology and a solar electric generating system (SEGS). As revealed,
both ISCC-DSG and ISCC-HTF present a high net efficiency, but ISCC-DSG
is the best option due to high temperature of superheated steam produced
in the receiver, which is economically viable since no additional
investment is required compared when using oil as HTF. Rovira et al.
[8] compared ISCC technology using HTF with DSG and showed that the
performance of ISCC- DSG is improved compared to HTF that requires an
additional steam generator. Another promising technology of
concentrating solar energy is the solar tower (ST) which is expected to
lead in the future. A comparative study [9] between Solar Rankine
Cycle (SRC) and ISCC plants based on PTC/ SPT system and a molten salt
to transfer heat to the water loop in SRC and to the synthetic oil
(Therminol VP-1) in ISCC showed that, with the same aperture area of the
solar field, the coupling with PTC caused low performance since less
quantity of solar energy is intercepted compared to heliostat field.
Abdel Dayem et al. [10] simulated the Kuraymat (Egypt) ISCC using
TRNSYS and compared the predicted thermodynamic performance with the
measured data under the same conditions of design specifications and
weather, and concluded good agreements. Aldali et al. [11] studied
the thermodynamic performance of ISCC system under the Libyan weather
where two operation modes (fuel saving and power boosting) with the same
solar field area, and concluded a reduction in fuel consumption and
CO2 emission. Dersch et al. [12] found that the
integration of PTC technology with CC plant provides an interesting way
for solar electricity generation, in addition to the environmental and
economic benefits. Montes et al. [13] showed the benefit of coupling
the solar field to CC, evidently in a hot dry climate such as in
Las-Vegas and Almeria where the good coupling of solar thermal power
made ISCC to operate efficiently and the cost of solar electricity gets
less. Antonanzas et al. [14] found that the solar hybridization with
CC installed through Spain has increased the electricity production in
the peak hours as well as the overall thermal efficiency and reduced
CO2 emission. During high ambient temperatures
coinciding with the period of higher normal radiation the steam produced
by the solar collectors alleviated the drop in electricity production by
CC plants during the peak demand periods and improved the overall
efficiency.
Besides the thermodynamic study the economic assessment is required from
the point of view of viability for electricity production. Various
previous techno-economic studies have investigated the performance of
ISCC technology, where the Levelized Cost of Energy (LCOE) is more
appropriate criterion. Price et al. [15] quantified the cost
reduction potential of LCOE for different configurations of solar energy
integration. A comparative investigation to a reference 50 MW power
plant with the solar mode only without a storage has revealed that the
substitution by an ISCC may reduce the cost of electricity by 33% (0.11
to 0.073 $/kWh), while the increase of concentrator size from 50 m to
150 m reduces the cost from 0.11 to 0.10 $/kWh. Horn et al. [16]
investigated the technical and economical aspects of ISCC installation
in Egypt; therefore a comparative study between ISCC using PTC
technology and solar tower technology was carried out and allowed to
conclude that from the point of view of electricity cost generation and
environmental effect PTC is still an attractive technology. Similarly,
Hosseini et al. [17] assessed the technical and economic aspects for
six different sizes of ISCC power in Iran. Based on LCOE and when the
environmental effect is considered, they concluded that ISCC using 67 MW
integrated to CC is the best choice for the construction of the first
solar power plant in Iran. Also, Mokheimer et al. [18] made a
techno-economic comparative study to integrate three types of CSP
technologies (PTC, Linear Fresnel Reflector (LFR) and ST) with a
conventional GT cogeneration plant of different power 50-150 MW under
Dhahran (Saudi Arabia) weather conditions, where THERMOFLEX with PEACE
software were used. They concluded that LFR technology is the optimal
configuration of solar integration with the steam side of GT
cogeneration with 50 MWe output. Duan et al. [19] proposed a novel
solar integration with CC (HRSG with two pressure levels) using PTC
technology and a part of compressed air from the compressor is used as
HTF through the receiver of solar PTC field. This novel ISCC using
compressed air has more advantages in terms of performance and economy
compared to ISCC using oil as HTF. Li et al. [20] presented a novel
integration of solar energy into CC with two different pressure and
temperature levels in HRSG using concentrating and non-concentrating
solar systems which are PTC using DSG system and Evacuated Tube (ET).
These two types of solar-collectors in a temperature cascade are shown
to contribute positively in the power plant performance in terms of
solar heat conversion, efficiency and lowering LCOE compared to ISCC-DSG
system power plant.
The present study concerns the thermodynamic and economic assessments of
ISCC power plants integrating an HSSG and using solar thermal energy
collected by HTF through PTC without any TES to reduce the cost and to
evaluate their operation and efficiency under the Algerian Sahara
climate with the solar radiation varying during the day and year. HSSG
was chosen to operate as a boiler in parallel to HRSG for enhancing the
quantity of steam generated during the sunny periods, thus only an
evaporator section is used and subsequently avoids extra preheating and
superheating exchangers. As the heat exchangers are the important
subsystems, the method of pinch point and approach point are used in the
thermodynamic modelling. The obtained results show prospective results
and viability of such technology which also may serve to support and
guide further installations.
THERMODYNAMIC MODELLING
As shown by Fig.1, this ISCC consists of a solar field via PTC, two GT
units and Steam Power Turbine (SPT) fed by two HRSG with a simple
pressure level in addition to HSSG working as an evaporator. The solar
energy is transferred to HTF (synthetic oil Thermonol VP-1 which has a
proven maturity in all solar thermal power plants, of a temperature
range of 13 - 395°C limited by the thermal stability [21]) in the
absorber tube and via an intermediate oil-to-water/steam heat exchanger
is transmitted into the Rankine cycle. During the cloudy periods and
nights the power plant operates as a conventional CC, while in the sunny
periods one part of feed water is withdrawn from HRSG and converted into
saturated steam by HSSG and then returned to HRSG where it is mixed and
superheated. The supplement of solar thermal energy provides an increase
in steam mass flow of the Rankine-Hirn cycle.