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.