Thermodynamic cycles for solar thermal power plants: A review

Solar thermal power plants for electricity production include, at least, two main systems: the solar field and the power block. Regarding this last one, the particular thermodynamic cycle layout and the working fluid employed, have a decisive influence in the plant performance. In turn, this selection depends on the solar technology employed. Currently, the steam Rankine cycle is the most widespread and commercially available option, usually coupled to a parabolic trough solar field. However, other configurations have been implemented in solar thermal plants worldwide. Most of them are based on other solar technologies also coupled to a steam Rankine cycle, although integrated solar combined cycles have a significant level of implementation. In the first place, power block configurations based on conventional thermodynamic cycles—Rankine, Brayton, and combined Brayton–Rankine—are described. The achievements and challenges of each proposal are highlighted, for example, the benefits involved in hybrid solar source/fossil fuel plants. In the second place, proposals of advanced power block configurations are analyzed, standing out: supercritical CO2 Brayton cycles, advanced organic Rankine cycles, and innovative integrated solar combined cycles. Each of these proposals shows some advantages compared to the conventional layouts in certain power or source temperature ranges and hence they could be considered attractive options in the medium term. At last, a brief review of proposals of solar thermal integration with other renewable heat sources is also included.


| INTRODUCTION
The thermal use of solar radiation has two main applications: it can be used directly as heat, both at domestic and industrial level (solar heat for industrial processes, SHIP); and it can be used in solar thermal power plants (STPPs) for electricity production. The total capacity of STPPs worldwide is 9267 MW e at the end of 2020 according to SolarPACES (2021), divided in turn into 6128 MW e of operational power, 1547 MW e under constructions and 1592 MW e , under development.
A STPP includes, at least, two main systems: the solar field and the power block. There are basically four concentrating solar technologies that can be coupled to a power cycle: linear Fresnel collector (LFC), parabolic trough collector (PTC), central receiver (CR) systems, and parabolic dish (PD) (Zarza-Moya, 2018). Regarding the power block, it is common knowledge that the cycle performance is directly affected by the maximum cycle temperature, improving as that temperature increases. Nevertheless, the solar field efficiency is lower as the working temperature increases, as the heat loss also increases. Therefore, a thermal optimization is necessary to optimize the global efficiency of the STPP (Breeze, 2016). Besides that, there are several technological challenges associated with high working temperatures: materials, the working fluid (degradation in oils or corrosion in molten salts) and limitations due to the solar technology itself (Mehos, 2017), as will be explain later.
Regardless the concentrating technology used, STPPs powered only by solar energy, show several important drawbacks: the need of large extensions for the concentration mirrors, due to the low energy density of the solar irradiation; lack of dispatchability as a consequence of the discontinuous nature of solar radiation; and the usual requirement of an intermediate medium to transfer the thermal energy to the working fluid of the power block (except in direct absorption receivers). The last drawback leads to a limitation on the maximum cycle temperature, due to the maximum allowed temperature in the material, which is lower compared to temperatures reached in combustion.
Regarding dispatchability, STPPs usually include a third important component, a thermal energy storage (TES) that allows the energy surplus to be stored for its subsequent management, thanks to the solar multiple higher than 1 (oversizing of the solar field). There are several storage technologies: thermocline tank, dual-tanks with a high density fluid (e.g., molten salts) or particles (Rovense et al., 2019), phase change materials (PCMs) and solid storage in bedrocks, this latter one suitable when the working fluid is a gas (Steinmann, 2015).
Another possibility to improve the dispatchability is to arrange a hybrid layout with auxiliary boilers, natural gas, or biomass (Powell et al., 2017). The hybridization provides a double benefit: the improvement in the management of the plant and the possible increase in the working temperature at the power block inlet and, therefore, the upgrade of the STPP global efficiency. If the hybrid configuration is based on a biomass boiler, the renewable nature is kept.
In summary, the main advantage of the STPPs is its sustainability and renewable nature, while the main drawbacks are the requirement of large land area for the concentrating mirrors, the dispatchability and the high cost of the technology compared to other energy sources.
STPPs can be classified according to different criteria, mainly the type of thermodynamic cycle the power block is based on, the solar field technology, and the type of heat transfer fluid (HTF) employed. This work focuses on the analysis of different configurations of the power block, describing the state of the art and its evolution over time, and putting forward advanced proposals. Section 2 is devoted to a brief description of the four concentrating solar technologies usually employed in STPPs. In Section 3, the conventional configurations in operational plants are described and classified according to the basic thermodynamic cycle employed: Rankine, Brayton, or combined Brayton-Rankine. Finally, some advanced proposals are described in Section 4, in search of solutions to increase efficiency and achieve lower generation costs. These proposals are either in a conceptual development state or in a prototype phase, but preliminary research studies show some advantages over the conventional configurations under certain conditions, and hence, they could be considered attractive options in the medium term.

| CONCENTRATING SOLAR TECHNOLOGIES FOR STPPs
This section describes the four concentrating solar technologies usually employed in STPPs, paying special attention to the maximum temperatures they can provide.
The PTC technology is the most common option in the commercially developed STPPs (Fern andez-García et al., 2010). Consist of a set of linear collectors with one-axis solar tracking and medium-high concentration factor (between 50:1 and 100:1) that allows to reach maximum temperatures as high as 600 C approximately. The HTF in the solar field, which transfers the solar heat to the power cycle, is usually synthetic oil, although it can also be molten salts, water-steam in the case of designs of direct steam generation (DSG) or even air. The maximum working temperature of PTCs is limited by the HTF degradation temperature-400 C in the case of Therminol VP1-, or by the selective coating of the absorber tube-550 C in advanced tubes- (Montes et al., 2010). This technology is very mature and there are multiple commercial designs, as well as different technologies for the absorber tube in the receiver.
CR plants consist of a field of heliostats, with two-axes solar tracking that concentrate solar radiation onto a receiver (Romero et al., 2002) that can be designed as: (i) direct-absorption receiver on solid particles or a fluidized bed; (ii) atmospheric or pressurized volumetric receiver; (iii) or indirect-absorption receiver by means of an intermediate surface to transfer the thermal energy to the working fluid. The concentration factor is higher than in PTC, between 200:1 and 1000:1, yielding to higher temperatures in the receiver. Thus, the thermal efficiency of the receiver is lower than in PTC while the conversion efficiency in the power cycle is higher, which may overcome the decrease of the receiver efficiency. The search for innovative HTFs able to work at high temperatures has become an important research field. Among the most promising ones, stand::advanced molten salts (Benoit et al., 2016;Turchi et al., 2018), pressurized gases ( Avila-Marín, 2011Ho et al., 2014), liquid metals (Pacio & Wetzel, 2013), and solid particles (Chen et al., 2021;Ho, 2016).
A LFC system consists of linear mirrors with one-axis solar tracking, with maximum concentration values similar to those of a PTC, although lower at yearly-average basis. These systems were developed later than the PTC and CR technologies. LFC presents some advantages related to the land requirements and robustness. Lower investment, operation, and maintenance costs can lead to savings of 11% in the electricity production (Morin et al., 2012). In addition, LFC technology has many degrees of freedom in both the optical and thermal designs, which can be optimized . Over a period of time, while the number of plants under construction was lower than that of PTC and CR technologies, the number of studies was higher, due to the clear economic advantages of LFC. However, the promising potential of CR technology has reduced again the interest on LFC, which has remained in a marginal niche compared to CR (Wang, 2019).
Finally, the PD technology, with the highest concentration ratio, is suitable for driving small engines, commonly stirling or micro-gas turbines, using air as the HTF. These systems allow large-scale generation (hundreds of MW e ) by replicating as many power unit as required (Hafez et al., 2016).

| Solar plants based on Rankine cycle
3.1.1 | Steam Rankine cycle solar plants Steam Rankine cycles (SRCs), in several regenerative and reheating layouts, have been widely used in fossil or nuclear thermal plants. The steam at the turbine inlet is usually superheated in the first and saturated in the second ones. These cycles generally work with pressures below the critical pressure. The first STPPs were based on this conventional scheme, coupling a PTC solar field to a SRC. SEGSs (Solar Electric Generation Systems) plants, built in California in the 1980s, are an example of them. Figure 1 shows the layout of SEGS-VIII and SEGS-IX plants, that is very similar to current PTC plants. The power block is a regenerative SRC with reheat. Superheated steam at the turbine inlet is at 371 C and 100 bar, and the reheating conditions are 371 C and 17.2 bar (Lippke, 1995).
At the early stages of STPP deployment, the research was focused on improving the solar field performance (Montes et al., 2009). Despite of keeping a conservative power block configuration, some optimization studies were carried out, for example, the optimal number of extractions or the influence of different cooling options in the condenser (Blanco-Marigorta et al., 2011;Deng & Boehm, 2011).
Currently, the SRC is the most widespread and commercially available power block option, either coupled to a PTC solar field working with thermal oil, and generating steam at 370-390 C and 100 bar or coupled to a CR solar field working with molten salts and generating steam at 550-600 C and 180 bar.
In this type of STPPs, solar-to-electricity efficiencies are around 25%, since the power block is limited and its thermal performance is in a range between 35% and 38% and the solar field efficiency is around 65%.

| Organic Rankine cycle solar plants
When the temperature of the heat source is in a low-to-moderate range (80 C< T max <300 C), organic Rankine cycles (ORCs) are regarded as a suitable option. Organic fluids can condense at pressures above the ambient one and have low boiling conditions that make them especially adequate to operate at low temperatures and pressures, either in subcritical or transcritical cycles, depending on the specific organic fluid selected.
ORC installations are smaller than conventional SRCs, due to the higher density of the organics fluids compared to water; and simpler, as a consequence of the thermodynamic behavior of numerous organic fluids, that present a positive gradient of the saturated vapor curve in the temperature-entropy diagram (dry fluids), as seen in Figure 2. This behavior implies that the ORC expanders can operate with saturated conditions at the inlet, increasing the mean heating temperature and ensuring that the expansion proceeds and finalizes in the vapor region. This property involves a twofold technological benefit: superheating is not always required to avoid humidity in the expander, although it may present advantages in some cases; and the thermal heat associated to the expander outlet can be used to the recuperative preheating of the liquid, without the need for more complex steam bleedings from the turbine (Figure 1). Therefore, compared with water, the selection of dry organic working fluid brings significant benefits in terms of a reduction of costs, bound to the absence of humidity. The lack of moisture increases the turbine thermal efficiency and reduces its maintenance costs (Desai & Bandyopadhyay, 2016).
It is important to highlight that in order to select a suitable dry fluid for a specific use, the cycle performance is more favorable when the fluid critical temperature is higher than the maximum cycle temperature or, at least, not much lower (Desai & Bandyopadhyay, 2016;Lai et al., 2011). Provided that there is not availability of dry or isentropic fluids with critical temperatures above 400 C, together with suitable condensing pressures, and keeping in mind the decisive influence of turbine inlet temperature over cycle efficiency, ORCs have an important limitation, compared to SRCs, when the heat source implies working fluid temperatures close to 400 C or higher.
F I G U R E 1 Simplified scheme of the steam Rankine cycle coupled to a parabolic trough solar power plant. This layout is similar to SEGS-VIII, SEGS-IX, and current plants (Montes et al., 2009) In consequence, ORCs have been generally proposed to be coupled to low-medium temperature renewable sources (from 80 to 300 C) and for limited power rates, like biomass, geothermal, heat recovery, and nonconcentrated solar systems (Braimakis & Karellas, 2017;Zhai et al., 2016). Manufacturers have been present on the market since the beginning of the 1980s, with many plants installed worldwide that use the low-medium temperature heat sources mentioned above. Currently, this technology is being also proposed for concentrated solar systems, although its share is still very low, with less than 1% of the total installed power. According to (SolarPACES, 2021), there are three commercial gridconnected plants, located in Arizona (1 MW e ), Morocco (3 MW e ), and Denmark of (12 MW e ) (Macchi & Astolfi, 2017;Rodríguez et al., 2016).
The research on the performance of ORCs coupled to a concentrating solar technology as heat source has been very active lately. In , authors highlight its suitability for low-moderate temperatures. Petrollese and Cocco (2019) also evaluate a recuperative ORC of 630 kW e , conected to a LFC solar field and operating under different scenarios of HTF mass flow and temperature. Subsequently, in (Petrollese et al., 2020), they study a STPP based on an ORC coupled to a LFC and a concentrated photovoltaic (CPV) solar field.
There are also various studies addressing the performance of ORC plants with power rates more similar to those of plants based on the Rankine steam cycle. For example, in Desai and Bandyopadhyay (2016), a thermo-economic evaluation model is developed to analyze the behavior of a 1 MW e ORC coupled to two different solar technologies, namely, PTCs and LFRs. The use of several working fluids (i.e., natural hydrocarbons, siloxanes, R245fa, and R113) is analyzed, including water as fluid for comparison purpose. In this study, when using PTC as solar field, the organic working fluid that results in the lower levelized cost of energy (LCOE) is R113, while the use of Toluene implies the highest cycle efficiency (31.2%). However, the former presents environmental problems, whereas the latter costs are still significantly high. The LCOE results for water compete closely with those of R113. In the case of selecting LFC, Toluene also presents the highest efficiency (28%) and LCOEs are slightly higher for all 12 fluids analyzed. To sum up, it can be concluded that ORCs are a good option in the case of low-medium power plants (less than 2 MW e ) and distributed generation. ORCs working with dry fluids offer higher nominal and off-design efficiencies at temperatures lower than 400 C, compared to SRCs. In those power and temperature ranges, steam Rankine plants lose the advantage of its higher efficiency, characteristic of high power steam Rankine plants. The higher capital cost per kW of SRCs compared to ORCs, at the considered power rates, is another drawback to be thought through.

| Brayton cycle solar plants
The coupling of solar energy to Brayton cycles is relatively new and less mature compared to Rankine-based cycles. The main advantage of Brayton cycles over Rankine ones is the simpler and lighter installation as steam Rankine facilities F I G U R E 2 Layout of a recuperative superheated organic Rankine cycle and T-s diagram are complex, with large equipment, such as the bulky condenser. However, the Brayton cycle presents lower efficiency when operated with medium temperature heat sources, as it is the case of PTC solar technology with synthetic oil. Really, to attain a competitive performance, the maximum temperature should be higher than 400 C up to 1000 C. Therefore, they are generally proposed for high concentration systems like CR or PD (Figure 3a), where the combustion chamber is replaced by the concentrated solar receiver. Hybrid fossil-solar configurations have been also proposed, requiring systems of lower solar concentration ratios. The solar contribution may be employed, either to preheat the combustion air, adding up to the recuperative contribution, as shown in Figure 3b, or to increase the production of water-steam in parallel with the exhaust gas source in steam injection gas turbine, for power and efficiency augmentation (Livshits & Kribus, 2012).
Brayton solar plants coupled to CR systems are intended for medium-high power levels. For example, Rovense et al. (2019) propose a design for a plant of 20 MWe based on a regenerative closed air Brayton cycle, with intercooled compression, joined to a pressurized air CR. This design allows a turbine inlet temperature of 800 C and the possibility to integrate a high temperature TES system. On the other hand, PD concentration systems are proposed for distributed electricity generation (Meas & Bello-Ochende, 2017;Semprini et al., 2016). The integration of a micro gas turbine with a solar dish has been analyzed as a promising option for several end use applications, in a power range between 100 kW e and 1 MW e (Al-attab & Zainal, 2015).
Finally, there is another configuration based on a closed Brayton cycle, characterized by the use of CO 2 as working fluid, which has also been proposed for concentrated solar power (CSP) applications (Kumar & Srinivasan, 2016), among others. Since this option is still in a low technological readiness level, a detailed description of its special features will be included in Section 4, devoted to innovative advanced configurations.

| Solar combined cycles
Combined cycle gas turbine (CCGT) technology had an important development and implementation for high power generation plants, that began at the 1990s. The heat recovery from the exhaust gas is used to generate steam in a Rankine bottoming cycle, which entails a high global energy conversion efficiency. From the beginning of its commercial deployment, the possibility of solar integration has been analyzed, either the solar-only option, or the option with fossil hybridization. However, as the fossil heat source is introduced in the topping gas turbine cycle, the solar-only alternative entails the replacement of the combustion chamber by the solar receiver, and the use of high concentration solar systems, as explained in the previous section. Therefore, the option of fossil hybridization has been preferred.
There is a wide consensus in the technical literature regarding the synergies between fossil and solar technologies. Since the production of conventional combined cycle plants decreases those days/hours of high solar radiation, due to the higher ambient temperature, the fossil-solar hybridization can take advantage, because it is just when the solar field performs best Zhu et al., 2015). Thus, the yearly operation comes up with higher values of solar-toelectric efficiency. The term ISCC (integrated solar combined cycle) generally refers to the particular configuration of combined cycles with solar integration into the Rankine bottom cycle, as shown in Figure 4. This configuration was initially proposed by the company Luz Solar International, and took advantage of the previous expertise gathered by the commercial operation of the SEGS plants, being nowadays the most used solar combined cycle configuration. Early studies proposed an ISCC plant with two gas turbines and a SRC. The solar energy was incorporated in parallel to the boiler, by means of heat exchangers that evaporated the preheated water before returning to the steam drum (Allani et al., 1997). Those early studies discussed the advantages of the system, established the parameters of the boilers, designed the heat exchangers in the heat recovery steam generator (HRSG) and the solar generator, and carried out initial economic analysis. Finally, the economic unfeasibility of the layout, in its contemporary costs scenario, was highlighted and, therefore, it was concluded that there was need for economic incentives (Kane et al., 2000;Kane & Favrat, 1999).
At the beginning of this century, the research in ISCCs increased, mainly due to the installation of some facilities in developing countries such as Argeria, Egypt, or Morocco, granted by the Global Environment Facility Agency. These studies were focused on the economic feasibility and the production cost of different layouts (Dersch et al., 2004;Mechthild Horn et al., 2004).
Nowadays, several ISCC plants have been installed, some of them thanks to the above-mentioned grants. Among them, stand plants installed in the Aïn Beni Mathar (Morocco), Hassi R'mel (Algeria), Kuraymat (Egypt), Martin Next Generation Solar Energy Center (USA), Archimede (Italy), and Yazd (Iran). There are other plants planned or under construction, such as Agua Prieta II in Mexico, Al-Abdaliyah in Kuwait or Duba 1, and Waad Al Shamal, both in Saudi Arabia.
Most of the plants, built or under development, consist of configurations with a solar field that provides heat in parallel with the HRSG, as mentioned before. PTC technology is the most used technology in ISCCs (Dersch et al., 2004;Franchini et al., 2013), and the solar energy is transferred to the water/steam using an additional steam generator, fed by synthetic oil coming from the solar field (T max = 390 C), except for Archimede, in which the HTF is a molten salt (T max = 550 C; Falchetta et al., 2009). Therefore, solar energy contributes to evaporate water, like in Hassi R'Mel and Yazd plants (Behar et al., 2011), although in some plants, solar heat provides a certain degree of steam superheating (Aïn Beni Mathar and Kuraymat) and water preheating (Archimede).
Other solar concentration technologies have been also considered in theoretical studies, namely, CR (Reyes-Belmonte et al., 2016, 2019 or LFC . For example, Manente et al. (2016) compare ISCC plants using F I G U R E 4 Layout of integrated solar combined cycle based on parabolic trough collector  PTC, LFC, and CR, concluding that it is not necessary a solar concentration factor as high as in CR to achieve 30% solar-to-electric efficiency. Although the use of a HTF is more reliable compared to DSG, due to the difficult of controlling the two-phase fluid in the solar field, the latter has been also studied, considering the advantage of not requiring an additional steam generator. For example, Rovira et al. (2018) compare the annual performance of ISCCs using the three solar concentration technologies, PTC, LFC, and CR, in every case with DSG in parallel with the high pressure evaporator of HRSG. Results show better performance in the case of PTC for both locations analyzed, with solar-toelectric efficiency up to 37%.
When the plant includes a HRSG with 2 or 3 pressure levels, usual in conventional CCGT plants, a very important issue is the selection of the optimal point in the cycle to integrate the solar energy. Many works have addressed this analysis; for example, Calise et al. (2018) carry out a dynamic study of an ISCC with solar integration in the lowpressure level of the HRSG. Brodrick et al. (2017) study the behavior of the ISCC layout with integration in the intermediate-pressure level, and Li and Xiong (2018) working with DSG, propose to incorporate the solar heat simultaneously in parallel with both evaporators, at the high-pressure and the low-pressure levels. Similarly, Bonforte et al. (2018) analyze the case of integration at the three pressure levels, including a management system to distribute the solar heat source between the three evaporators. They conclude that the installation cost increase significantly, whereas the fuel saving of the proposal is negligible. Rovira et al. (2013) compare four different layouts of integration in a dual pressure HRSG, considering preheating and superheating as well as evaporation, both with HTF and DSG. They work out that a lower HRSG irreversibility is reached when solar heat is used for evaporation at the high-pressure level, compared to water preheating. Finally, Mabrouk et al. (2018) propose a layout that initially includes 11 heat exchangers, each of them in parallel with the corresponding HRSG exchanger, but placed in series regarding the HTF flow. Those exchangers are selected o discarded, deciding the optimal network for different values of the solar thermal power. The performance study concludes that solar integration in high temperature exchangers is the most favorable choice.
If the ISCC is specifically designed for boosting operation (solar plus full heat recovery), the turbine, as well as the superheaters and economizers of the HRSG, must be oversized. That design would imply a lower turbine efficiency during nonsolar irradiation periods, partially compensated by a higher steam production in the oversized heat exchangers. Besides that, lower ambient temperatures, that could occur during nonirradiation periods, benefit gas turbine performance, also mitigating the previous effect. However, if the plant is designed to operate in a fuel-saving mode, this oversizing would not be necessary. To perform a consistent analysis, this issue must be taken into account, when comparing conventional CCGT and ISCC performances.

| Combined cycles with solar integration into the gas turbine
Although the most common scheme of solar integration in CCs is the solar heat supply into the SRC, particularly at the high-pressure level (ISCC technology), the option of integration into the gas turbine has been explored as well. Some layouts regarding the integration into the Brayton cycle have been already described in Section 3.2. In the case of CCTG, for example, Amelio et al. (2014) propose to heat up the combustion air by passing it through PTCs, managing without an intermediary HTF. Thus, the pressurized air coming out of the compressor is sent to the solar field where is preheated up to 580 C, prior to enter the combustion chamber. As expected, the fuel required to achieve a predetermined turbine inlet temperature is reduced. The authors estimated a fossil fuel saving of 22% at design conditions, and 15.5% evaluating the annual performance. Duan et al. (2017) propose a configuration that integrates solar contribution to preheat the combustion air, but with the peculiar feature of a prior use of the air exiting the compressor to preheat water, that is then incorporated to the HRSG. Although the air is thus previously cooled, the temperature achieved, the solar contribution is finally higher, which results in a greater power generation and fossil fuel saving. Other designs also propose the use of a CR, to preheat the combustion air (Okoroigwe & Madhlopa, 2016). Rovira et al. (2018) compare the annual performance of a reference CCTG with the performance of two ISCC layouts that differ in the solar heat integration option: a conventional ISCC scheme, in which solar heat is used to directly evaporate water (DSG) at the high pressure level of the SRC, and a second scheme in which the solar heat is used to preheat the Brayton cycle combustion air. In both cases, three different solar concentration technologies (LFC, PTC, and CR) are analyzed. Results show that ISCC with combustion air preheating suffer a reduction in yearly energy production in comparison to the reference CCTG, as a consequence of the pressure drop in the solar heat exchanger. On the contrary, DSG increases the yearly production. Nonetheless, the former option entails notable higher solar-to-electricity efficiencies, with values above 40% in the case of PTC and CR.

| SOLAR PLANTS BASED ON ADVANCED POWER CYCLES
4.1 | Innovative organic cycles 4.1.1 | Balanced hybrid Rankine-Brayton cycle  proposed a configuration named balance-hybrid Rankine-Brayton (B-HRB) cycle for moderate temperatures heat sources, in a range between 350 and 400 C, as it is the case of moderate concentration factor solar technologies. In that study, the proposed cycle was compared to several other cycles and/or different working fluids, among them, different configurations of SRCs, ORCs (Acetone, R125), and sCO 2 cycles. As can be observed in Figure 5, the configuration implies the hybridization of a transcritical Rankine with a Brayton cycle, combination that allows a low and constant temperature for heat rejection and a high mean temperature for heat supply. This configuration also includes a single recuperator and a compressor to divert a fraction of the total mass flow in order to achieve a quasi-balanced behavior of the recuperator, and very low irreversibility. These features lead to a potentially high cycle efficiency, as well as a simpler facility, but require the use of a working fluid with specific properties. For example, dry organic fluids, such as isobutane, propane, and R125, fulfill those requirements, namely: high critical temperature, that allows condensation at adverse high ambient conditions; and low critical pressure (roughly below 25% of the maximum fluid pressure), which implies a more constant specific heat during the heating process . As a consequence, the B-HBR cycles working with either isobutane, acetone or R141b, reach higher efficiencies than the transcritical ORCs analyzed under the boundary conditions of the study. In comparison with SRC, efficiencies are very much alike, but with the considerable advantage of being a less complex facility, that includes a single recuperative heat exchanger instead of the typical regenerative SRC layout (Figure 1). In , the study was extended to analyze off-design operation, assuming that heat supply comes from a PTC solar field, with a maximum temperature of 397 C. The annual performance simulation is based on hourly meteorological data, corresponding to Almeria (Spain). Among the various working fluids analyzed, the study concludes that propane and R125 are the most suitable, even under adverse conditions. In the case of propane, cycle efficiency varies along the year between 41.37% and 30.2%, whereas in the case of R125 cycle, efficiencies are slightly lower.
As depicted in the temperature-entropy diagram of this cycle ( Figure 5), the working fluid temperature at the inlet of the heat source exchanger (state point 4) is quite high, which in turn implies a high HTF return temperature to the heat source system, that is just what is desirable to work with so-called closed sources. That is the case of the PTC solar fields, that require a synthetic oil return temperature above a minimum value to guarantee a high solar thermal efficiency.

| Unconventional organic Rankine cycles (RDE and DRDE)
The B-HRB cycle described in the previous section is not adequate to be used as power block in heat recovery applications (open heat sources), where it is desirable to considerably reduce the temperature of the heat transfer fluid, associated to the waste heat source, to recover as much residual thermal energy as possible. Having this in mind,  propose a new configuration, derived from the previous B-HRB, but more suitable for this kind of application. This configuration may have a role in solar plants with novel designs, as will be explained in the following section. Figure 6 presents the layout and temperature-entropy diagram of the proposal that consist of a Rankine cycle with two heating lines, owing to the split of the flow exiting the pump in two streams. The main one is heated by the heat source, and then proceed to the expander or turbine. Given that the inlet temperature of this stream is very low (state point 2), it is possible to recover a large percentage of the thermal energy from the main source. A secondary stream makes use of the thermal energy associated to the main expander exhaust by means of the recuperator. Since the fluid leaving this recuperator at supercritical pressure has a high thermal energy (point 6), a secondary expander is incorporated downstream, thus increasing the power generation. Considering that this layout includes a single recuperator and two expanders, it will be referred to as recuperated and double expanded (RDE) cycle. Propane has been identified as a very good option as the working fluid in this cycle.
When the thermal heat of the secondary expander exhaust is significant (point 7), a secondary recuperator could be used to preheat the main stream before entering the source heater; configuration that could be more convenient to prevent excessively low HTF exhaust temperatures that may entail, for instance, acid condensation problems. This configuration is named double recuperated and double expanded cycle (DRDE), and it has been proposed as the bottoming cycle in the configuration shown in Figure 9, that will be explained in Section 4.3.3.

| Supercritical CO 2 Brayton cycle
In recent years, the use of CO 2 operating at supercritical conditions (sCO 2 ) in recuperative closed Brayton cycles, has gained notable consideration for power generation. This may be imputed to the specific characteristics of CO 2 , with relatively low values of both temperature and pressure at supercritical conditions (30.98 C, 73.77 bar) and its very high density, that implies a much smaller size of the equipment involved. In the case of the compressor, if the inlet conditions are selected near the critical point (e.g., pressures between 75 and 90 bar and temperatures between 35 and 55 C) the power required for compression is very low in comparison to the power generated by the turbine expansion. This characteristic, together with a recuperative configuration, allows to achieve values of thermal cycle efficiency even higher that those obtained with conventional superheated SRCs, with a simpler configuration. Two important drawbacks must be mentioned: the lack of proven-commercially available technology for equipment working with supercritical CO 2 (turbomachinery and heat exchangers); and, the peculiar behavior of CO 2 specific heat near the critical point, very dependent both on pressure and temperature. This latter feature makes it difficult to design the recuperator, F I G U R E 6 Recuperative double expansion cycle (RDE) and T-s diagram (p 1 ¼ 1:2 MPa; p 2 ¼ 17 MPa; working fluid propane; Rovira, Muñoz, S anchez, et al. (2020)) because the specific heat of the two streams (i.e., the high temperature-low pressure stream and the low temperaturehigh pressure stream), are very different. This problem is overcome by means of the use of a second recuperator (low temperature recuperator [LTR]) and an auxiliary compressor (re-compressor). At the exit of the high temperature recuperator (HTR) the flow is split, diverting a fraction through the re-compressor. The lower mass flow that circulates through the higher pressure-lower temperature side of the (LTR) allows the balance of the specific heat capacities of both streams. Figure 7 presents the layout and temperature-entropy diagram of the recompression supercritical CO 2 cycle.
It can be said that the recompression sCO 2 power cycle is one of the most studied cycles lately. As the maximum temperatures of the working fluid are in a range from 500 C up to 1000 C, this cycle achieves thermal efficiencies that can compete advantageously with other conventional options (Kumar & Srinivasan, 2016;Rovira et al., 2014;. Apart from CSP, this technology has been proposed for nuclear plants, geothermal systems, fuel cells and high temperature heat recovery systems. In relation with different sCO 2 Brayton configuration options, Zhu et al. (2017) analyze the recompression configuration together with other sCO 2 layouts that do not include the bypass recompressor; instead they incorporate a precompressor or partial cooling. Coco-Enríquez et al. (2017) compare the performance of a solar plant, based on a SRC, with four solar sCO 2 cycles configurations, all of them with reheating: the basic regenerative cycle and three recompression layouts (the standard, the partial cooling, and the intercooling). In this work, several cycle parameters are optimized by means of multivariable algorithms, for example: the bypass fraction, the main compressor total pressure ratio and the intercooling pressure. The study concludes that a considerable efficiency enhancement is obtained, compared to the conventional Rankine cycle option, if a recompression sCO 2 cycle is selected as power block. Other studies carry out optimization analyses of the recompression cycle parameters, aiming to increase the cycle efficiency. Each study considers a specific solar technologies, mainly CR. In this line,  optimize a recompression cycle with reheating; Binoti et al. (2017) optimize and compare additional layouts, considering main compressor intercooling, partial cooling, and the conventional recompression cycle; Monjurul et al. (2020) study the off-design and annual performance of a standard recompression cycle coupled to CR, comparing dry and water cooling; and Chen et al. (2021) compare six different configurations, four of them with recompression, also considering the essential aspect of the off-design performance. In this case the STPP includes a dry cooling system, a particle-based high temperature receiver, and uses hot particles as the storage medium in TES. In this study the recompression cycle and the simple regenerative cycle obtain a better off-design performance.
F I G U R E 7 Recompression sCO 2 cycle layout and corresponding T-s diagram, in this case: p 1 ¼ 8:1 bar; p 2 ¼ 20 MW;α ¼ 0:31 An important issue to consider is the high value of the maximum pressure in sCO 2 cycles. The compression ratio must be higher than 2.5 to achieve an optimum cycle efficiency. That means pressures as high as 200 bar or even 300 bar, and heat exchangers that must work with a high pressure difference between both streams. For this reason, printed circuit heat exchangers (PCHEs), designed to support a high mechanical stress, are commonly selected. However, their design imply the flow through very small channels, so clogging problems can arise when using, for example, molten salts as heat transfer fluid. To overcome this problem, Linares et al. (2020) propose a novel layout where the heat power is supplied downstream of the turbine and, therefore, at the low pressure side of the cycle (typically 80-90 bar). This option allows the use of a shell and tube design for the source heat exchanger when molten salts are used, as it is often the case of CR solar systems.
Lately, important research programs, as the Solar Power Gen3 Demonstration Roadmap from the National Renewable Energies Laboratory (NREL; Mehos, 2017) or the Australian Solar Thermal Research Initiative (ASTRI) (Gurgenci et al., 2014), have selected the recompression sCO 2 cycle for the power block of CR systems. The objective is to achieve efficiencies higher than 50% while using simpler facilities, thus lower LCOE values. Numerous studies establish that it is possible to reach that high efficiency with some of the layouts previously described and referenced, as long as the inlet turbine temperature reach a value of at least 700 C. This value, that appears modest in a context of fossil Brayton cycles, entails a challenge in the case of CSP. CR plants in operation use conventional molten salts (nitrate salts) as HTF for the receiver and the TES, coupled to a conventional SRC. The HTF operational temperature is 565 C, therefore below the maximum of 600 C that guarantees the stability of the conventional solar salts. However, CR operating with sCO2-Brayton cycle requires the use of alternative high-temperature salts, able to operate at temperatures up to 750-800 C. At present, chloride ternary salts are considered a promising option; anyhow, in addition to adequate thermophysical properties, the potential salt options must show good compatibility with possible alloys to manufacture the equipment (particularly the receiver and TES hot tank) with essential properties, such as: sufficient strength, corrosion resistance, as well as a non-negligible issue-admissible cost. Another promising technology to overcome the problem to operate at 700-800 C maximum temperatures, is the use of solid particles in the receiver and TES, that has been already mentioned in some of the referenced studies (e.g., Chen et al., 2021). In this case, also arise quite a few of technological challenges, for example, efficient particle heating, fluidized-bed flow control, materials to endure abrasion, among others.
In search of other options to increase efficiency, the integration of a bottoming ORC cycle to recover the rejected heat of the sCO 2 cycle has been explored, although the advantage of simplicity is compromised in that case. Along that research line, Song et al. (2018) explore the potential of adding a bottoming ORC in the sCO 2 cycle to improve its thermal performance, carrying out a parametric optimization of that combined cycle, and Singh and Mishra (2018) analyze a solar PTC plant feeding a similar combined cycle (a recuperative sCO 2 and bottoming ORC). In both cases the sCO 2 cycle do not include recompression and the maximum temperature is lower than 400 C. Hou et al. (2018) carry out the optimization of a combined sCO 2 recompression cycle (T max = 750 ) with a regenerative ORC using a zeotropic mixture fluid. On the other hand, Mohammadi et al. (2020) propose a layout for a hybrid CR-GT-sCO 2 solar plant, reaching high maximum temperatures (1000 C). The gas turbine exhaust gases feed two sCO 2 cycles in series: a recompression cycle followed by a sCO 2 partial cooling cycle. This configuration obtains lower LCOE, because the sCO 2 cycles include smaller components, expected to entail lower costs than those required in steam Rankine bottoming cycles. However, the global thermal efficiency obtained was lower.

| Innovative configuration proposals for integrated solar combined cycles
4.3.1 | Integrated solar combined cycle using gas turbine with partial recuperation The idea of CC with a recuperative gas turbine has been explored in several works, always in search of layouts that would increase plant thermal efficiency (Carcasci & Facchini, 2000;Franco & Casarosa, 2002). However, this configuration has not been implemented in commercial operating plants, because the advantages are not particularly relevant. The improvement in gas turbine efficiency is balanced out by the lower temperature of the exhaust gases that feed the HRSG, downstream of the recuperator. That lower temperature entails a lower steam production, leaving aside the greater complexity of the layout. However, further studies follow this research line, but based on the concept of partial recuperation and extended it to ISCC (Liu et al., 2018;Rovira et al., 2017;. Having in mind the high solar-to-electricity efficiency obtained with the integration of the solar heat in the gas turbine, Rovira, Abbas, S anchez, et al. (2020) propose a layout of ISCC plant where the gas turbine integrates a heat exchanger for Partial Recuperation (ISCC-PR). This layout, showed in Figure 8, aims to save an amount of fuel equivalent to the solar input, without requiring any variation in the turbine inlet temperature.This proposal keeps the integration of the solar source in the bottoming cycle, as in standard ISCC, with the advantage of using this reliable technology that ensures a constant steam production. However, in this proposal the gas turbine exhaust is not sent in full to the HRSG. In periods of solar irradiation, when solar heat contributes to the high pressure water evaporation, a fraction of the exhaust gases is sent to the gas turbine recuperator, thus achieving a reduction of fuel consumption. That fraction reduces its temperature and must be introduced downstream in the HRSG, at the high pressure superheater inlet, where it is mixed with the main stream. When the partial recuperation takes place, the gas mass flow through the HRSG is affected, and therefore, the fraction sent to the recuperator must be modified in each case, to maintain the steam mass flow generated, as well as its temperature. This proposal achieves a solar-to-electricity efficiency above 50%, using a proven technology for solar integration in the bottoming cycle, as in conventional ISCC (Rovira, Abbas, S anchez, et al. (2020)).

| Integrated solar combined cycle configurations based on ORC as the bottoming cycle
As other alternatives for ISCC, Chacartegui et al. (2009) propose a low temperature ORC, instead of SRC, as the bottoming cycle in medium and large power combined cycle plants. They conclude that while modern conventional CCGT plants make use of gas turbines in the topping cycle with very high inlet temperatures, to achieve an efficiency close to 60%, combined cycle plants including a toluene ORC as bottoming cycle may reach similar global efficiency with a more moderate turbine inlet temperature, that entails inferior values of NO X emissions, and lower manufacture and maintenance costs.
In this research line, Cao et al. (2016) study the coupling of a ORC cycle to a low power gas turbine (12 MW e ) and Shaaban (2016) analyze the performance of a peculiar solar integrated combined cycle plant including two low temperature cycles: a SRC and a ORC. The SRC is fed in the conventional way, by both heat sources: the solar heat and the gas turbine exhaust. However, as in this proposal the gas turbine includes compressor intercooling, the ORC gets its heat source from the cooler rejected heat. In (Zare & Hasanzadeh, 2016), authors analyze a configuration with two low temperature ORCs and a recuperative gas turbine as topping cycle.
F I G U R E 8 Layout of integrated solar combined cycle with partial recuperation in gas turbine (Rovira, Abbas, S anchez, et al., 2020)

| Other integrated solar combined cycles
These proposals seek to obtain new and more advanced hybrid configurations to integrate and manage both heat sources: the solar source and the gas/biogas. The aim is to develop ISCC plants where, if possible, the solar contribution becomes more important than the gas contribution. In , different layouts with and without solar integration, are analyzed and compared. This study includes a novel cycle that combines a topping recuperative gas turbine, aiming a reduction in fuel consumption, with an unconventional ORC (DRDE), as shown in Figure 9. The recuperative configuration of the gas turbine entails a drop of at least 100 C in the turbine exhaust temperature, which in turn means a reduction of the bottoming cycle power. The annual performance of all the configurations considered is carried out in two different locations: Almeria and Las Vegas. These two locations present different values of the mean annual ambient temperature (lower in Almería) and solar irradiation (higher in Las Vegas), and those parameters have a significant influence in the annual performance.
The ISCC-R-DRDE presents a superior performance compared to the state-of-the-art CCGT and to ISCC, with a higher annual mean efficiency. The results show that the fuel saving is more important in the colder site (4% vs. 2.5%), whereas the solar irradiation level does not have a significant effect. From the thermodynamic point of view, the authors conclude that the combination of a recuperative gas turbine and a double recuperative double expansion organic cycle shows promising results.

| PROPOSALS OF INTEGRATION WITH OTHER RENEWABLE HEAT SOURCES
Several studies propose the integration of the solar source with other renewable sources. For example, Cakici et al. (2017) analyze the performance of a supercritical regenerative ORC that integrates a geothermal heat source with the heat from a solar field of PTCs. Jiang et al. (2017) consider those two renewable energy sources, geothermal and solar, each of them individually coupled to a sCO 2 recompression cycle, but with an integrated operation: the base-load power is supplied by the geothermal plant whereas the solar thermal plant generates supplementary power to cover the peak electricity demand. In relation to hybridization with biomass, Pantaleo et al. (2017) propose a combined cycle composed of a biomass external fired gas turbine and a superheated recuperative ORC. The novelty of this proposal is the use of a thermal storage system between the topping and the bottoming cycle, and the integration of a solar field of F I G U R E 9 Layout of integrated solar combined cycle with recuperative gas turbine and double recuperation & double expansion (DRDE) as bottoming cycle  PTCs connected in parallel with the thermal storage. Morrone et al. (2019) study a proposal of a transcritical ORC driven by a PTC solar field and a conventional biomass boiler connected in series. The boiler operates if the solar radiation does not satisfy the energy request. The study concluded that the biomass integration raises the annual net solar-to-electric efficiency. Finally, integration with waste heat coming from industrial processes has also been analyzed. For example, Bellos and Tzivanidis (2018) study the performance of an ORC driven by a solar field of PTCs, with synthetic oil as HTF and an intermediate storage tank. In this proposal, the waste heat of low-medium grade temperature (150-300 C) is mainly supplied to the economizer of the ORC steam generator. However, it may contribute to the evaporation process, depending on the waste heat temperature and the working fluid employed.

| CONCLUSION
This work has been focused on the analysis of different configurations of the power block in STPPs, describing the state-of-art and its evolution over time; putting forward advanced proposals, highlighting their drawbacks and their challenges. Currently, the SRC is the most widespread and commercially available power block option, usually coupled to a PTC solar field working with synthetic oil. The hybridization is a valuable option, because it involves a double benefit: the improvement in the management of the discontinuous solar source, and the possible increase in the maximum temperature of the working fluid in the power block, yielding higher STPP global efficiency. ISCC plants are good examples of these advantages, and several projects worldwide have chosen this configuration. In this review, several advanced alternative layouts of solar integrated combined cycle plants have been described (e.g., ISCC-PR, ISCC-R-DRDE), proposed to further increase the plant thermal efficiency with a better management of both heat sources, solar and fossil, and, if possible, increasing the solar source annual contribution. As CR technology is gaining prominence, other power block options, that require high maximum working fluid temperatures, have been considered, as it is the case of sCO 2 Brayton cycles. This solar-only proposal presents the advantage of a high efficiency with a simpler facility compared to conventional options. Other configurations, suitable to operate with moderate temperature heat sources (ORCs, B-HRB) have been described as well, presenting promising results in specific ranges of temperature and power, showing cycle efficiencies comparable to the SRCs but with the noteworthy advantage of being a less complex facility.
In summary, it can be stated that the current solar thermal electricity scenario consists of multiple available alternatives, both for solar technologies and for power conversion, including hybrid systems. An economic assessment and comparison of the different alternatives has not been addressed, because the results provided by authors, although promising, involve high uncertainties, as new technologies need to make use of some components still under development and not yet available commercially. Among the challenges to promote the advances power cycle, stands the development of the involved turbomachinery and heat exchangers, which should work with nonconventional fluids and/or under unconventional ranges of pressure, so currently their technological readiness level is low. Besides that, there are several additional technological challenges associated with operation at high working temperatures (800 C), concerning CRs and the TES systems. Different technologies are being proposed to tackle the problem, each one with their own challenges, and in all the cases it is essential to find alternative HTFs with adequate properties, compatible with materials to manufacture the equipment at an admissible cost.

ACKNOWLEDGMENT
This work has been developed in the frame of the ACES2030-CM project, funded by the Regional Research and Development in Technology Programme 2018 (ref. P2018/EMT-4319) and supported by the Spanish Ministry of Economy and Competitiveness through the PID2019-110283RB-C31 project.

CONFLICT OF INTEREST
The authors have declared no conflicts of interest for this article.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.