17
Solar power towers using supercritical CO2 and supercritical steam cycles, and decoupled combined cycles
Abstract
Solar power-tower systems (also known as central receiver systems) can efficiently achieve high temperatures because of the high concentration ratios they can achieve using different configurations of the collector field and receiver. The combination of solar power towers with high-temperature cycles permits to increase in the global efficiency in the conversion of solar radiation to electricity with respect to concentrated solar power (CSP) plants based on the sub-critical Rankine cycle and could result in levelized cost of energy (LCOE) reduction, as far as the increase in efficiency outweighs the increased costs associated with the use of more expensive equipment and materials. Although operating temperatures higher than 1000°C can be achieved with solar power towers, there are still significant technological barriers that must be overcome before CSP plants operating at these elevated temperatures reach the market. The use of supercritical power cycles operating at temperatures in the range 600–800°C, only moderately higher than those of the current CSP plants, has been identified as a promising path to increase the efficiency and reduce the LCOE of the next generation of CSP plants that would require technological developments achievable in the short to medium term. Another option to increase the efficiency of concentrated solar thermal (CST) plants that requires only incremental technology developments is the concept known as decoupled solar combined cycles, based on a high-temperature topping cycle whose rejected heat is used to charge a thermal energy storage system which, in turn, feeds a bottoming cycle. The concepts presented in this chapter are excellent candidates to become the next generation(s) of commercial CST plants, although in some cases significant technology developments are required.
Keywords
Brayton cycle; Central receiver systems; Decoupled combined cycles; Power cycles; Solar power towers; Supercritical carbon dioxide; Supercritical steam Rankine cycle
17.1. Introduction
Solar power-tower systems (also known as central receiver systems) can efficiently achieve high temperatures because of the high concentration ratios they can achieve using different configurations of the collector field and receiver. The combination of solar power towers with high-temperature cycles permits to increase the global efficiency in the conversion of solar radiation to electricity with respect to today's CSP plants based on the sub-critical Rankine cycle and could result in LCOE reduction, as far as the increase in efficiency outweighs the increased costs associated to the use of more expensive equipment and materials.
Although operating temperatures higher than 1000°C can be achieved with solar power towers, there are still significant technological barriers that must be overcome before CSP plants operating at these elevated temperatures reach the market. The use of power cycles operating at temperatures in the range 600–800°C, only moderately higher than those of the current CSP plants, has been identified as a promising path to increase the efficiency and reduce the LCOE of the next generation of CSP plants that would require technological developments achievable in a relatively short term.
The technological developments required to improve the technology readiness level of these options seem to be achievable in the short term at least in some cases, although, as of 2016, none of these concepts has been demonstrated beyond a pilot scale. In some cases, the adaptation of the state-of-the-art equipment would be sufficient, while in other cases further research is required.
In any case, this path has been recognized in different programs and initiatives, like DOEs Sunshot [16], the European Solar Thermal Electricity Industry Association, ESTELA, [12] or the Australia Solar Thermal Research Initiative, ASTRI [26].
In this chapter we present a review of three options to follow this path:
• power towers with supercritical steam Rankine cycles (SSRCs);
• power towers with supercritical CO2 cycles;
• decoupled solar combined cycles (DSCCs).
17.2. Solar power towers with supercritical cycles
In this section, the different options for the integration of solar power towers and supercritical steam and CO2 cycles are described and discussed. Both supercritical cycles operate in temperature ranges only moderately superior to state-of-the-art solar power towers, although the different physical and chemical properties of the cycle working fluid involve specific requirements for the solar power tower in terms of integration schemes, heat transfer media (HTMs), materials, and so on.
17.2.1. Supercritical steam Rankine cycles
SSRCs operate at pressures greater than 22.1 MPa in a range of temperatures only moderately higher (600–720°C, up to 760°C in the case of advanced ultra-supercritical cycles) than those of the state-of-the-art solar power towers and have been identified as a promising option to increase the efficiency and reduce the cost of the electricity generated by CSP plants in the near or medium term.
The Sunshot Initiative of the US Department of Energy [16] identified the possibility to adapt current molten salt and direct steam generation (DSG) solar power towers to supercritical steam cycles operating in the range 600–700°C. Years before, the long-term scenario for 2020 analyzed by [37] already considered a solar power tower plant using an SSRC. Kolb [23] explored the benefits of a new generation of molten salt CSP plants with SSRCs operating in the range 600–650°C, finding potential LCOE reductions of up to 8% compared to current sub-critical molten salt plants. [39] studied the cost reduction potential of ultra-supercritical steam cycles—350 bar and up to 720°C with a thermal efficiency of 55%—coupled with a solar tower using tubular receivers with different liquid metals and salt mixtures as HTMs. According to their estimates, the LCOE could be reduced up to 15% compared to the current molten salt CSP plants. The same authors [40,41] compared the potential of two innovative receiver concepts—internal direct absorption, beam down—with tubular receivers using different HTMs and ultra-supercritical steam cycles, estimating potential LCOE reductions from 7.2% (direct absorption receiver with chloride salt) to 0.5% (beam down with molten nitrate salts) with respect to molten salt power towers of 2016. Peterseim and Veeraragavan [34] compared three solar power towers using an advanced steam cycle with sub-critical parameters and two supercritical solar power towers: the first, a hybrid solar–natural gas configuration with state-of-the-art molten salts and steam parameters of 280 bar and 620°C and the second, a solar-only plant with a precommercial, advanced molten salt and steam parameters of 620°C at 280 bar. They found that the LCOE could be reduced about 4.3% with the third option, that requires the development of a commercial molten salt mixture, stable at about 700°C. [38] analyzed the requirements, in terms of materials technology, for the use of advanced ultra-supercritical steam cycles operating at steam conditions up to 760°C with up to 35% improved efficiency compared to superheated steam cycles of 2016.
However, working at high temperature and pressure increases the costs of materials and equipment. In addition, there are some challenges associated to the development of SSRC solar power towers:
• Size of the plant. A major challenge for the immediate deployment of SSRC solar power towers is the upscaling of current power tower plants or downsizing the SSRC. The largest solar power tower built today is Crescent Dunes (Nevada, USA) with a rated power of 110 MW and 10 full-hour molten salt thermal energy storage (TES); on the other hand, the smallest commercially available supercritical steam turbines are in the range of the 250 MW. Hybrid solar–fossil fuels options may provide a path to overcome the current gap [34].
• High-temperature receivers. State-of-the-art molten salt receivers operate at temperatures of about 560°C. Increasing this temperature requires the development of new salt mixtures which are stable at the temperatures required by SSRC plants (600–700°C) or the use of other HTMs like solid particles, liquid metals, air, or supercritical steam. On the other hand, materials that can withstand the demanding conditions during the lifetime of the plant will be required for the receiver, heat exchangers [35], and other equipment. Further technology developments are required in all cases.
17.2.1.1. Integration of solar power towers and supercritical steam Rankine cycles
There are multiple possible configurations of SSRCs solar power towers: direct or indirect supercritical steam generation, direct, indirect, or thermochemical storage system, solar-only or hybrid plants, and so on. In the following, we provide an overview of the main variants referenced in the literature.
Direct steam generation
In the DSG configuration, water enters the solar receiver at low temperature and supercritical pressure and is heated to temperatures above 550°C. No additional heat exchanger or steam generator is required unless the plant includes an indirect-type TES. The main concern for the development of direct supercritical steam receivers is the demanding requirements in terms of materials and design to withstand the high-pressure and high-temperature conditions of the steam [19]. An early 2010s project in CSIRO [3,15] has demonstrated the feasibility of such receivers, with a pilot project (309 kW thermal power) generating supercritical steam at pressures from 22.5 to 23.5 MPa and temperatures of 570°C (Fig. 17.1).
Indirect steam generation
Another option to integrate solar power towers with supercritical steam is to use a high-temperature HTM. In this case, the concentrated solar flux is used to increase the temperature of the HTM and a high-pressure HTM steam generator is used to generate the supercritical steam feeding the turbine. The overall configuration of the plant can be very similar to current molten salt plants (Fig. 17.2). Potential HTMs are molten salt mixtures, liquid metals, solid particles, and so on.
Figure 17.1 A picture of the CSIRO supercritical steam receiver [15].
Figure 17.2 Sketch of a supercritical steam plant with liquid HTM and TES [41].
Molten salt mixtures appear today as the most immediate option for SSRC solar power towers. A hybrid configuration based on the current molten salt plants producing supercritical steam at 280 bar and 545°C with additional natural gas superheating to reach a steam temperature of 620°C was analyzed by Peterseim and Veeraragavan [34] (Fig. 17.3).This design would not require any major technological development since only the molten salt—steam generator would need to be modified to operate at supercritical pressure.
The use of other salt mixtures that have stability beyond 650°C would allow to increase the steam temperature without the use of natural gas superheating. Chlorides, carbonates, and fluorides have been identified as potential candidates [19,40,41]. However, operating beyond 650°C will require the development of new receiver materials and, in some cases, the use of oxygen blankets to avoid or reduce the rate of corrosion in the hot storage tank and other equipment [23,37].
Figure 17.3 Process diagram of the supercritical solar tower with natural gas superheating. From Peterseim JH, Veeraragavan A. Solar towers with supercritical steam parameters—is the efficiency Gain worth the Effort? Energy Procedia 2015a;69:1123–32. http://dx.doi.org/10.1016/j.egypro.2015.03.181.
Liquid metals and metal eutectics can operate at the high temperatures required to generate supercritical steam. They exhibit excellent heat transfer characteristics allowing for high efficiency and high flux density on the receiver. The use of alkali metals (mainly sodium) and lead–bismuth eutectic (LBE) has been analyzed by different authors [19,31]. Table 17.1 presents a comparison of the physical properties of solar salt (state-of-the-art molten nitrate salt mixture), sodium, and LBE. While the upper temperature limits of both liquid metals are well above the current range of supercritical and ultra-supercritical steam plants, sodium has the drawback of its extreme reactivity with air and water and LBE has a low thermal capacity, preventing their application in direct TES configurations.
Solid particles (see Chapter 10 of this book) can achieve high temperatures too. They can be used in different types of receivers and configurations as HTM and TES medium—direct absorption falling particles [43], fluidized bed [25], particles in tube [13], and so on—and have the potential to be relatively cheap. In principle, the integration of solid-particles solar power towers with supercritical steam cycles seems straightforward, with an overall scheme similar to that of molten salts solar power towers. However, the solid-particle technology in its different variants is still at a relatively early stage of development [19].
Table 17.1
Comparison of solar salt and two candidate liquid metals
| Physical property | Solar salt | Liquid Na | Liquid LBE |
| Lower temperature limit,°C | 220 | 98 | 125 |
| Upper temperature limit,°C | 600 | 883 | 1670 |
| Heat capacity cp, kJkg−1 k−1 | 1.52 | 1.27 | 0.143 |
| Thermal conductivity λ, Wm−1 k−1 | 0.53 | 69.8 | 13.7 |
| Density ρ, kg m−3 | 1804 | 850 | 10,139 |
| Dynamic viscosity μ, MPa s | 1.69 | 0.27 | 1.44 |
| Prandtl number | 4.85 | 0.005 | 0.015 |
| Other characteristics | Low-cost direct TES is possible | React with air and water | Larger density, lower cp |
From Pacio J, Singer C, Wetzel T, Uhlig R. Thermodynamic evaluation of liquid metals as heat transfer fluids in concentrated solar power plants. Applied Thermal Engineering 2013;60:295–302. http://dx.doi.org/10.1016/j.applthermaleng.2013.07.010.
Figure 17.4 Schematic of ammonia-based thermochemical energy storage system [24].
Ammonia-based thermochemical energy storage
Ammonia-based thermochemical energy storage (TCES), Fig. 17.4, is based on the realization of the endothermic dissociation of NH3 at the solar receiver–reactor absorbing solar energy. The products of the reaction, N2 and H2, are stored in a physical tank (charging process). The thermochemical energy stored in the tank can be used by performing the reverse, exothermic reaction at the discharge reactor (steam generator). A prototype ammonia-based TCES coupled to a parabolic dish was demonstrated at the Australian National University, ANU, achieving temperatures of 475°C.
The use of ammonia TCES in SSRCs has been explored by several researchers [9,24], exploring alternatives for the physical gas containment (salt caverns, drilled underground shafts), supercritical steam generator–reactor, and the solar reactor. The authors expect to develop a TCES with a cost of US $15/kW h thermal.
17.2.2. Supercritical carbon dioxide Brayton cycles
Supercritical carbon dioxide (sCO2) closed-loop Brayton cycles have been introduced and described in Chapter 11 of this book. sCO2 cycles have the potential to provide higher efficiency than the sub-critical steam cycles used in CST power plants and equivalent or higher than supercritical steam cycles operating on the same temperature range, with the advantage of lower operating pressure and greater compactness. In addition, the heat rejection temperature ranges of sCO2 cycles make them appropriate for the use of dry-cooling. Sensible heat TES can be easily integrated because the CO2 presents a single phase in all the cycle processes. These characteristics reveal a potential to develop highly efficient and compact CST plants (Fig. 17.5).
Different sCO2 cycle configurations have been discussed in Chapter 11. The selection of the most appropriate one for integration in CST towers depends on the cycle efficiency, the temperature difference, and the complexity—which is directly associated to its cost.
Chacartegui et al. [7] compared two configurations of sCO2 Brayton cycles—simple and recompression—and an sCO2–ORC combined cycle, integrated with central receiver systems. Their analysis, which used a simple solar tower model, concluded that the recompression cycle provides the best performance. Other analyses using an effectiveness model for the recuperator [42] suggested that the performance of the partial cooling and recompression cycles were similar. A later study using a conductance model for the recuperators [27] showed that the partial cooling cycle provides higher efficiency than the recompression cycle up to high values of the recuperator conductance. Assuming that the cost of the equipment increases with the recuperator conductance, the partial cooling configuration is better for low- and medium-conductance values. In addition, they also found advantages for the partial cooling cycle regarding the design and operation of the solar receiver. The simple sCO2 Brayton cycle has a lower efficiency than the partial cooling and the recompression cycle, but has the advantage of its simplicity for near-term implementation.
Figure 17.5 Simple closed-loop supercritical carbon dioxide Brayton cycle [2].
Iverson et al. [21] modeled and validated—within the limits of the experimental setup available—an sCO2 cycle for CST plants. The results include the transient part-load response of the cycle and the identification of necessary research for successful implementation of sCO2 Brayton cycles in CST power plants, including the development of turbines, bearings, seals, heat exchanger designs, and materials.
Padilla et al. [32,33] compared the thermal and exergetic performance of four cycle configurations: simple, recompression, partial cooling, and recompression with main compression intercooling configurations, finding that the latter has the best performance with a thermal efficiency of about 47% at temperature greater than 700°C.
17.2.2.1. Integration of solar power towers and supercritical CO2 cycles
As in the case of SSRCs, the options for the integration of sCO2 cycles and solar power towers are multiple: direct or indirect sCO2 generation, direct, indirect, or thermochemical storage system, solar-only or hybrid plants, and so on. In the following, we provide an overview of the main variants referenced in the literature.
Direct sCO2 receivers
The main advantage of direct receivers, where the working fluid is the same at the solar receiver and power cycle, is that it eliminates the need for intermediate heat exchangers, thus avoiding the thermal and exergy losses and the cost associated to this equipment. In the case of solar powers with sCO2 cycles, there are three main options for a direct sCO2 receiver: tubular, pressurized volumetric, and fluidized bed, small particle-gas receiver. Of these three options, only the first seems to be sufficiently mature for its commercial deployment in the near to medium term.
Tubular receivers would operate at pressures of up to 30 MPa. Ortega and Christian [30] established the design requirements for tubular CO2 receivers operating at supercritical conditions. They have also developed a coupled optical–thermal-fluid model [29] and performed a structural and creep–fatigue evaluation of such receiver [28]. Their results show that thermal efficiencies close to 85% can be achieved at the receiver when using appropriate aiming-point strategies to obtain adequate flux profiles on the receiver surface and flow patterns with high recirculation of the working fluid.
Besarati et al. [4] proposed a direct CO2 solar receiver based on compact heat exchanger (CHE) technology. The receiver consists of Inconel 625 plates with square shaped channels bonded together, operating at pressures close to 20 MPa, and temperatures between 530°C and 707°C with a solar flux density of 500 kW/m2.
Indirect receivers for sCO2
The integration of sCO2 receivers with storage is today a major, unsolved challenge; thermal [18] storage of supercritical fluids is not viable [22] and the use of a different TES medium does not seem to be an efficient option. In this context, the use of different HTMs as working fluid appears as the best option for integration of solar power towers with storage and sCO2 cycles.
As in the case of the SSRCs, the concentrated solar flux is used to increase the temperature of the HTM and a high-pressure HTM—CO2 heat exchanger is used to generate the high-temperature supercritical CO2 feeding the sCO2 turbine. The overall configuration of the plant can be very similar to molten salt plants (Fig. 17.6). Potential HTMs are, again, molten salt mixtures, liquid metals, and solid particles in different receiver configurations. Ho et al. [20] reviewed several high-temperature designs for sCO2 Brayton cycles, concluding that the most viable option today for indirect CO2 heating and TES is the use of falling particles receivers.
Liquid metals—mainly sodium and LBE—can operate at the high temperatures required by sCO2, but, again, sodium has the problem of its extreme reactivity with air and water and LBE has a low thermal capacity, preventing their immediate application in direct TES configurations.
Thermochemical energy storage for sCO2 cycles
Several thermochemical cycles for TES coupled to sCO2 cycles have been proposed. Buckingham et al. [5] propose redox transitions in metal oxides and sulfur-based cycles, where energy is stored inexpensively in the form of elemental sulfur (Fig. 17.7). These reactions that occur at temperatures between 500°C and 1000°C take place at the solar receiver (moving-bed reactor). The authors estimate that these configurations have the potential to increase the efficiency of the plant with respect to the state-of-the-art molten salt Rankine cycle plants, however, with significant uncertainties related to cost.
Figure 17.6 Schematic of a solar-driven, indirectly heated, closed-loop supercritical CO2 Brayton power cycle. From Ho CK, Carlson M, Garg P, Kumar P. Cost and performance Tradeoffs of alternative solar-driven S-CO2 Brayton cycle configurations. In: Proceedings of the ASME 2015 power and energy conversion Conference. San Diego, California, 2015. p. 1–10.
Calcium looping with sCO2 cycle is a promising option too, to be integrated with solar power towers [6]. The calcium looping process is based on the calcination–carbonation reaction of CaCO3. It has a high volumetric energy density and can produce temperatures close to 900°C, well coupled to sCO2 cycles and solar power towers.
17.2.3. Comparison of supercritical steam and carbon dioxide Brayton cycles
A review of high-efficiency thermodynamic cycles and their applicability to CSP systems performed by Dunham and Iverson [11] concluded that steam Rankine systems may offer higher thermal efficiencies up to temperatures about 600°C, material limits for steam components as of 2016 while an sCO2 recompression Brayton cycle may be the best candidate for higher temperature, with potential efficiency of about 60% at 30 MPa and above 1000°C and wet-cooling.
The benchmarking exercise realized by [8] concluded that a superheated steam Rankine cycle is both more efficient and more cost-effective than the three sCO2 power cycle concepts and the SSRC considered in their analysis (Figs. 17.8 and 17.9). However, the validity of these conclusions is limited by the fact that they considered a constant thermal power input of 213.7 MW—relatively small for state-of-the-art supercritical and combined cycles—and that the comparison was mainly based on the normalized cost ($/kW) rather than the LCOE.
In any case, any conclusions as of 2016 can be only considered of a temporary character, since there is a vast activity in the field of high-temperature materials for advanced power generation and other related fields that could modify the outcomes of these analyses.
17.3. Decoupled solar combined cycles
DSCCs have been identified as a promising option for LCOE reduction that takes advantage of the high temperature achievable by means of CSP systems and the use of TES. A DSCC is the combination of a high-temperature cycle where the heat is provided by a CSP system and a lower temperature, bottoming cycle. The heat rejected during the operation of the high-temperature cycle is used to charge the TES. The energy stored in the TES can be asynchronously used to feed the bottoming cycle, thus decoupling the operation of both cycles and allowing for great operation flexibility.
The DSCC concept seems “naturally” linked to the solar tower technology because of its capacity to efficiently achieve high concentration ratios and high temperature, thus taking advantage of the high exergy of the solar radiation.
The DSCC concept provides great flexibility in the design of the plant. Despite its relatively recent development—the first reference to DSCCs dates from 2012 [36]—several different configurations can be found in the literature.
Figure 17.8 Comparison of cycle performance at 20 MPa maximum pressure under (a) wet-cooling and (b) dry-cooling conditions. From Dunham MT, Iverson BD. High-efficiency thermodynamic power cycles for concentrated solar power systems. Renewable and Sustainable Energy Reviews 2014;30:758–70. http://dx.doi.org/10.1016/j.rser.2013.11.010.
Figure 17.9 Comparison of cycle performance at 30 MPa maximum pressure under (a) wet-cooling and (b) dry-cooling conditions. From Dunham MT, Iverson BD. High-efficiency thermodynamic power cycles for concentrated solar power systems. Renewable and Sustainable Energy Reviews 2014;30:758–70. http://dx.doi.org/10.1016/j.rser.2013.11.010.
Researchers of CENER (National Renewable Energy Center, Spain) identified and analyzed two configurations [36], both of them based on multitower solar fields, each tower having its own gas turbine (Brayton cycle). Both the multitower solar energy collection and the Brayton cycle have the potential to achieve high efficiency.
In the first configuration (concept A of the authors) the heat rejected from every gas turbine is used to charge a common, single medium-temperature thermal storage system that provides thermal energy to the bottoming cycle, a single low-temperature organic Rankine cycle (ORC). The potential of this configuration to achieve competitive LCOE lies in the use of the relatively low-cost TES (based on a thermocline tank with mineral oil) and the ORC. Because of the characteristics of both the TES and the ORC, this configuration could be used in relatively small CSP plants.
The second configuration (concept B) combines the multitower system with a single high-temperature molten salts TES and a superheated Rankine cycle. The TES is charged trough air–molten salt heat exchangers (one per tower). The emphasis of this configuration is on the efficiency achievable by the combination of the Brayton and superheated Rankine cycles and the high TES capacity, allowing for large CSP plants.
In both cases, the authors consider a solar energy collection system with a heliostat field, a beam-down reflector, and a secondary concentrator coupled to a ground-based high-temperature air receiver (Fig.17.10).
The authors analyze two cases (“conservative” and “optimistic”) for each configuration:
• Concept A. Both the optimistic and the conservative Brayton cycles have a rated power of 13.06 MW and the ORC of 1.7 MW, with four towers and storage capacities of 13 (conservative) and 14 h (optimistic). The estimated LCOE are 11.7 c€/kWh and 9.9 c€/kWh (relative reductions of 26% and 41% with respect to the base case—Gemasolar CSP plant located near Seville, Spain—respectively).
• Concept B. The optimistic case has a Brayton cycle of 44.8 MW and 10 solar towers, while the conservative leads to a Brayton cycle of 53.7 MW and 12 towers. The Rankine cycle has a power of 19.9 MW in both cases. The preliminary results show potential LCOE reductions of 26–41% with respect to the reference case.
A variant of the first configuration is proposed and analyzed in greater detail by the same group [14]. This variant, based on already existing components, uses a medium-temperature superheated steam Rankine cycle (40 bar, 274°C) instead of the ORC. In this case, the system consists of 32 towers and a TES capacity of 9 full hours of the Rankine cycle of the solar field is composed of small, single-facet heliostats with a biomimetic layout arranged in a hexagonal shape. The pressurized air receiver with advanced cavity configuration operates at 800°C. This relatively low temperature is considered to minimize the risks and the efficiency penalties associated with higher temperature. The rated power of each Brayton cycle is 3.34 MW, and the common Rankine cycle has 10.8 MW. The authors use a probabilistic approach to assess the performance and estimate the achievable cost reduction, concluding that the LCOE could be reduced up to 25% from the reference case, based on the Gemasolar CSP plant, with a probability of 90% (Fig.17.11).
Figure 17.10 Schemes of the decoupled solar combined cycle, concept A (left) and concept B (right) [36].
Figure 17.11 Scheme of the DSCC concept proposed by [14] based on a multitower system with Brayton cycles and a central oil storage systems feeding a bottoming medium-temperature Rankine cycle.
Agalit et al. [1] propose a decoupled configuration with a Brayton cycle and a superheated-steam Rankine cycle, and two TES systems (Fig. 17.12). The solar field with a high-temperature air receiver (800–1200°C) is connected to the gas turbine and a high-pressure regenerator/packed-bed storage system using natural (quartzite) rocks. The system includes a fuel combustor which can be used to complement or replace the solar energy input. The combustor and the TES provide operational flexibility to the Brayton cycle. The exhausts from the gas turbine feed the second, low-pressure TES, also based on natural rocks, which in turn feeds the Rankine cycle through an air-steam heat exchanger. The authors focus their work in the simulation of the thermal storage systems, not providing any LCOE estimates.
This configuration is a modification of the SUNSPOT concept [17] developed at the Stellenbosch University of South Africa, which includes only the low-pressure TES and is conceived for partially decoupled operation (the Rankine cycle would operate during nigh time). The authors elaborate a relatively simple model which is used to calculate the LCOE, estimating values of €0.11–0.18/kWh.
Crespo [10] proposes a DSCC hybrid configuration, with a large fuel-driven Brayton cycle and a molten salt heat exchanger to recover the exhaust thermal energy of the air turbine, combined with a molten salt CSP plant operating at about 560°C (Fig. 17.13). The storage system can be charged either from the solar receiver or from the gas turbine exhausts at the same conditions. The generation of electricity from the steam cycle is completely decoupled either from the gas turbine or from the solar part. According to the author's estimates, the efficiency in the conversion of the thermal energy from the fuel to electricity is similar to that of conventional combined cycles, with solar shares (fraction of the electricity generated from solar radiation to total electricity generated) ranging from 50% (in base load operation) to 80%.
Figure 17.12 Scheme of the DSCC proposed by [1]. This concept includes a high-temperature regenerator/TES for the topping Brayton cycle and a second, low-pressure TES which feeds the bottoming Rankine cycle.
Figure 17.13 Hybrid DSCC concept. Proposed by Crespo L. Ste plants: beyond Dispatchability Firmness of Supply and integration with VRE. Energy Procedia 2015;69:1241–1248. http://dx.doi.org/10.1016/j.egypro.2015.03.161.
As a summary, the combination of a high-temperature power cycle with a bottoming cycle using a TES system in place of the conventional heat recovery steam generator not only allows to a very large extent the decoupling of the operation of the bottoming cycle from that of the high-temperature cycle, but it also introduces additional degrees of flexibility in the design and operation of solar tower systems that can be used to create more flexible, reliable, and cost-effective systems.
17.4. Summary and conclusions
In this chapter, the integration of SRCCs and supercritical CO2 Brayton cycles with solar power towers is reviewed. Both power cycles operate in temperature ranges only moderately superior to molten salt plants, being thus well suited to the temperature levels achievable with central receiver systems. The integration of these power systems poses, however, significant challenges both on the power cycle and on the solar energy collection system, especially in terms of the identification or development of new materials that can withstand the demanding operating conditions of these systems without increasing the costs so much that the potential efficiency increase is negatively counterweighed.
According to the literature, SRRCs can be a good option for temperatures up to 600–650°C, while sCO2 cycles would be more competitive for higher temperatures. However, there is no complete consensus about this conclusion, since most of the possible configurations require significant technology developments, especially in the solar energy collection and TES systems, whose outcomes are difficult to estimate in terms of performance and costs.
Thermochemical storage based on ammonia dissociation, metal oxides or sulfur redox reactions, or calcium looping coupled to supercritical plants also seem to be promising long-term options.
This chapter also includes a section on DSCCs, where the heat rejected by a high-temperature cycle integrated with a solar power tower is used to charge a TES system which, in turn, feeds a bottoming cycle. Several DSCC configurations are described and discussed. DSCCs have the potential to achieve high efficiency without the need of significant technology development, being an excellent candidate for the next generation of CST plants.
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