Fundamental Thermo-Economic Approach to Selecting sCO2 Power Cycles for CSP Applications

ScienceDirect Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect Energy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia 1876-6102 © 2017The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. The 15th International Symposium on District Heating and Cooling Assessing the feasibility of using the heat demand-outdoor temperature function for a long-term district heat demand forecast I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc aIN+ Center for Innovation, Technology and Policy Research -Instituto Superior Técnico,Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal bVeolia Recherche & Innovation,291 Avenue Dreyfous Daniel, 78520 Limay, France cDépartement Systèmes Énergétiques et Environnement -IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. The main scope of this paper is to assess the feasibility of using the heat demand –outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. Keywords: Heat demand; Forecast; Climate change Energy Procedia 129 (2017) 963–970 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 10.1016/j.egypro.2017.09.215 10.1016/j.egypro.2017.09.215 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 1876-6102 Available online at www.sciencedirect.com Energy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia IV International Seminar on ORC Power Systems, ORC2017 13-15 September 2017, Milano, Italy Fundamental Thermo-Economic Approach to Selecting sCO2Power Cycles for CSP Applications Francesco Crespi, David S´ anchez∗, Jos´ e M. Rodr´ ıguez, Giacomo Gavagnin University of Seville, Department of Energy Engineering, Camino de los descubrimientos s/n, 41092, Seville, Spain Abstract The interest in Supercritical Carbon Dioxide (sCO2) power cycles has grown exponentially in the last decade, thanks to distinctive features like the possibility to achieve high thermal efficiencies at intermediate temperature levels, small footprint and adaptability to a wide variety of energy sources. In the present work, the potential of this technology is studied for Concentrated Solar Power applications, in particular Solar Tower systems with Thermal Energy Storage. A thorough sensitivity analysis based on turbine inlet temperature and pressure ratio is done for twelve sCO2cycles, considering their effects on thermal efficiency and specific work, along with solar share and temperature rise across the solar receiver. The most important conclusions of this section are that: a) the peak values of these thermodynamic figures of merit are obtained at different pressure ratios; b) specific work and temperature rise across the receiver seem to follow parallel trends whilst this is not the case for thermal efficiency; c) for a given turbine inlet temperature, higher pressure ratios always increase the receiver temperature rise strongly, but the effect on thermal efficiency is uncertain as this can either increase or decrease, depending on the cycle considered. A deeper analysis of thermal efficiency and receiver temperature rise is therefore mandatory, given that these parameters strongly affect the capital cost of CSP power plants. On one hand, a higher thermal efficiency implies a smaller solar field, the largest contributor to the plant capital cost; on the other, the temperature rise across the receiver is inversely proportional to the size of the thermal energy storage systems, as it is also the case for state of the art steam turbine based CSP plants. An economic analysis is developed using an in-house code and the open-source software System Advisor Model to evaluate the trade-offs between these two effects. The results obtained for the two most representative sCO2cycles show somewhat unexpected results. c 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. Keywords: sCO2power cycle, Thermo-economic analysis, CSP power plant, Thermal Energy Storage, ∗Corresponding author. E-mail address: [email protected] 1876-6102 c 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. Available online at www.sciencedirect.com Energy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia IV International Seminar on ORC Power Systems, ORC2017 13-15 September 2017, Milano, Italy Fundamental Thermo-Economic Approach to Selecting sCO2Power Cycles for CSP Applications Francesco Crespi, David S´ anchez∗, Jos´ e M. Rodr´ ıguez, Giacomo Gavagnin University of Seville, Department of Energy Engineering, Camino de los descubrimientos s/n, 41092, Seville, Spain Abstract The interest in Supercritical Carbon Dioxide (sCO2) power cycles has grown exponentially in the last decade, thanks to distinctive features like the possibility to achieve high thermal efficiencies at intermediate temperature levels, small footprint and adaptability to a wide variety of energy sources. In the present work, the potential of this technology is studied for Concentrated Solar Power applications, in particular Solar Tower systems with Thermal Energy Storage. A thorough sensitivity analysis based on turbine inlet temperature and pressure ratio is done for twelve sCO2cycles, considering their effects on thermal efficiency and specific work, along with solar share and temperature rise across the solar receiver. The most important conclusions of this section are that: a) the peak values of these thermodynamic figures of merit are obtained at different pressure ratios; b) specific work and temperature rise across the receiver seem to follow parallel trends whilst this is not the case for thermal efficiency; c) for a given turbine inlet temperature, higher pressure ratios always increase the receiver temperature rise strongly, but the effect on thermal efficiency is uncertain as this can either increase or decrease, depending on the cycle considered. A deeper analysis of thermal efficiency and receiver temperature rise is therefore mandatory, given that these parameters strongly affect the capital cost of CSP power plants. On one hand, a higher thermal efficiency implies a smaller solar field, the largest contributor to the plant capital cost; on the other, the temperature rise across the receiver is inversely proportional to the size of the thermal energy storage systems, as it is also the case for state of the art steam turbine based CSP plants. An economic analysis is developed using an in-house code and the open-source software System Advisor Model to evaluate the trade-offs between these two effects. The results obtained for the two most representative sCO2cycles show somewhat unexpected results. c 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. Keywords: sCO2power cycle, Thermo-economic analysis, CSP power plant, Thermal Energy Storage, ∗Corresponding author. E-mail address: [email protected] 1876-6102 c 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 964 Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 2F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 Nomenclature sCO2Supercritical Carbon Dioxide. CS P Concentrated Solar Power ηth Thermal Efficiency WsSpecific Work [kJ/kg] TES Thermal Energy Storage ∆Tsolar Temperature Rise Across Solar receiver [oC] SS Solar Share TIT Turbine Inlet Temperature [oC] PR Pressure Ratio Tin,compr Compressor Inlet Temperature [oC] Pin,compr Compressor Inlet Pressure [MPa] ηpol,CCompressor Polytropic Efficiency ηpol,pump Pump Polytropic Efficiency ηpol,TTurbine Polytropic Efficiency ∆PHX Pressure Drops FLiNaK LiF-NaF-KF molten salt SAM System Advisor Model 1. Introduction The roots of supercritical Carbon Dioxide (sCO2) technology can be tracked back to the initial work by Sulzer [1] and Escher-Wyss even if the theoretical fundamentals are generally attributed to Angelino and Feher [2–4]. The technical challenges posed by closed cycle gas turbines, in particular when fluids other than air were considered, along with the early take-offof combustion turbines hindered the further development of closed cycle engines which, for the following forty years, were hardly considered for large scale applications. In the early 2000’s, the assessment of sCO2 power cycles for Gen IV nuclear reactors carried out at the Massachusetts Institute of Technology, in particular the work by Dostal [5], drew attention to the technology again and triggered a very intense research and industrial activity to explore the potential of these systems in diverse applications: nuclear [6], fossil fuel combustion in stand-alone oxyfuel [7,8] or combined cycle [9] systems, waste heat recovery [10] and Concentrated Solar Power plants. For the latter case, several studies have already suggested that sCO2systems have the potential to yield lower costs of electricity than state-of-the-art power plants based on steam turbine technology [11–15]. The present paper employs an approach already presented by the authors to analyse the potential of sCO2power cycles in CSP applications. This methodology relies on the fundamental thermodynamic features of the cycles, considered from a global standpoint for the entire design space, as opposed to the partial analysis of certain cycle layouts in reference cases that is usually found in literature. The concept is set forth in [2,16], showing the need to revisit the original works by Angelino and Feher [3,4] in order to establish the principles and a common framework upon which the technology can be developed further. Amongst the forty-two sCO2power cycles reviewed in [2], twelve layouts are selected in [16]. For these, the impact of pressure and temperature ratios on ηth and Wsis studied in order to assess their theoretic potential from a thermodynamic standpoint. The analysis is developed without direct limits to the value of peak pressure, other than those coming from the feasibility of internal heat recovery (recuperator performance). This yields a comprehensive, global approach to cycle performance even if sometimes based on ridiculously high peak pressure values. The approach also provides an insight into remaining efficiency gains in future systems with higher grade materials. Fig. 1(a) shows an example of the results provided by the methodology described above, when applied to the twelve cycles considered. It is observed that each line is made up of two branches with full (black) and empty (white) markers. These correspond to operating conditions involving peak pressures lower or higher than 40 MPa respectively, this threshold set arbitrarily as a representative value for present technology [16]. As said, the combination of both sets (affordable and unaffordable pressures) in one single plot enables an assessment of the remaining gains in performance, whether efficiency or footprint. The present paper applies the cited methodology to Concentrated Solar Power plants, focusing on the two cycle layouts shown in Fig. 1(b): Transcritical Simple Recuperated [3] and Supercritical Partial Cooling [22]. These Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 965 F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 3 (a) Comparison of cycles operating at TIT=750 oC [16]. (b) Cycle Layouts Fig. 1. Comparison of cycles operating at TIT=750 oC and layouts of the two configurations considered. layouts are selected from previous analysis based on their potential for CSP applications [2,16]. The original features of the analysis are presented in the following section. 2. Sensitivity Analysis The analysis in [16] is now applied to Concentrated Solar Power (CSP) plants incorporating Thermal Energy Storage (TES) systems operating on high temperature molten salts. To this aim, two additional figures of merit are considered: temperature rise of the molten salts across the solar receiver (∆TS olar) and solar share (SS). The former parameter is of interest for its strong effect on the size and cost of the storage system. The Solar Share (SS), defined as the ratio from the solar heat input to the total heat input to the power block, stands as the best index to measure the degree of renewable energy integration when hybrid (solar-fossil fuel) power plants are taken into account. 2.1. General Assumptions Three turbine inlet temperatures temperature levels are considered in this work: 800, 900 and 1000 oC. The first value1(800 oC) sets the peak temperature expected from next-generation power blocks integrated in commercial CSP plants with molten salts TES, and therefore represents the case of a solar-only system. At higher TIT, a complementary energy source (usually fossil fuel through combustion with either air or oxygen) is needed. For the sake of simplicity, the current analysis does not include specific models of this complementary heat addition but, rather, TIT is simply set to 900 or 1000oC arbitrarily depending on the case. This assumption does not affect the validity of the analysis, whose focus is on the thermodynamic potential of the layouts considered and not on the technical feasibility or other aspects such as the technology readiness level. The high temperatures taken into account are unattainable by the molten salts used in contemporary CSP plants, whose degradation temperature is in the order of 600 oC [17]. Therefore, a FLiNaK molten salt is adopted after a through literature review, given that its operating range is compatible with the sCO2power blocks considered. A comparison between these two salts, along with their thermodynamic properties, is provided in Table 1. In order to model the heat transfer between FLiNaK and sCO2, a constant temperature difference of 20oC between the hot (salt) and cold (carbon dioxide) fluids is adopted (note that CO2behaves close to ideally in this temperature and pressure region far from the critical point). 1Private communication with Abengoa Solar. 966 Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 4F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 Salt Composition Freezing Point [oC] Boiling Point [oC] Density [kg/m3] Specific heat J/kgK] Price [$/kg] Solar Salt NaNO3-KNO3220 [17] 600 [17] −0.636 ·T+2090 [18] 0.172 ·T+1443 [18] 1.1 [19] FLiNaK LiF-NaF-KF 454 [20] 1570 [20] 2408.9−0.624 ·T[21] 1267.2+1.0634 ·T[21] 8.6 Table 1. Comparison between a standard molten salt and FLiNaK. FLiNaK price calculated from data available in [21]. Table 2 provides the boundary conditions used for both cycles in Fig. 1(b), such as compressor inlet conditions, turbomachinery efficiency and pressure drops. It is worth noting that the compressor inlet temperature for the Transcritical Simple Recuperated cycle is set to 15oC in order to enable condensation. A more detailed discussion of the assumptions and models employed in the sensitivity analysis, along with a description of both cycles, is provided in [16]. Cycle Tin,compr [oC] Pin,compr [MPa] ηpol,T[%] ηpol,C[%] ηpol,pump [%] ∆PHX [%] Transcritical Simple Recuperated 15 5 90 89 83 1 Supercritical Partial Cooling 32 5 90 89 83 1 Table 2. Parameters used in the sensitivity analysis. Note that turbomachinery efficiency is polytropic. 2.2. Results A comparison of the Transcritical Simple Recuperated and Supercritical Partial Cooling cycles is presented in Figs. 2 to 5. It is easily observed that none of them achieves very high thermal efficiency nor specific work. Nevertheless, in spite of this similarity, their performances differ when ∆Tsolar and SS are considered: •The shape of the curves changes significantly depending on the configuration considered. This is best observed in Figs. 2(a) and 4(a) where the potential of the Supercritical Partial Cooling cycle to achieve high efficiency and specific work at lower peak pressures becomes evident. The Supercritical Partial Cooling cycle presents a much lower slope of the ηth −Wscurves, which becomes almost horizontal at 1000 oC turbine inlet temperature. This suggests that the performance of the Supercritical Partial Cooling cycle could be less affected by deviations from the optimal pressure ratio (PR), hence confirming the conclusion by Dostal in [22]. •A much stronger impact of turbine inlet temperature on the performance of the Supercritical Partial Cooling cycle is also visible in the same set of figures, even if this does not offset the superior performance of the cited cycle. •Moreover, a comparison between Figs. 3(a) and 5(a) also shows a quick drop in ∆Tsolar for the Supercritical Partial Cooling cycle whereas this parameter remains at a higher value in the Transcritical Simple Recuperated layout. This means that a small rise in PR can provide a larger increase in ∆Tsolar than in ηth, thus reducing the cost of the thermal storage system. •Figures 3(b) and 5(b) present the impact of cycle layout and design conditions on solar share. Interestingly, the Transcritical Simple Recuperated cycle seems able to achieve higher SS at turbine inlet temperatures above 800oC. More specifically, the difference is ∆SS≈5 percentage points in favour of the cited layout even if this is at substantially higher operating pressures. The first conclusion drawn from these figures is that, for given turbine inlet temperature, the maximum values of ηth,Wsand ∆Tsolar are obtained for different PRs in each cycle, Figs. 2 and 4. Moreover, peak performance (highest ηth and Ws) for the Transcritical Simple Recuperated cycle is always achieved above the technological limit of 40 MPa (empty markers) whereas the maximum ∆Tsolar is attainable at lower pressures (except for TIT=1000 oC). On the contrary, the Supercritical Partial Cooling cycle can virtually achieve the highest efficiency within the technological limits. Secondly, it worth noting that ∆Tsolar and Wsare almost directly proportional until the maximum values of ∆Tsolar = 346oC is reached. The same applies to the ∆Tsolar vs. ηth plots for the Transcritical Simple Recuperatated cycle in Fig. 3(a) but not for the Supercritical Partial Cooling layout in Fig. 5(a) which shows a different pattern. The horizontal Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 967 F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 5 (a) ηth vs Wsdiagrams for the Transcritical Simple Recuperated cycle. (b) ∆Tsolar vs Wsdiagrams for the Transcritical Simple Recuperated cycle. Fig. 2. ηth vs Wsand ∆Tsolar vs Wsdiagrams for the Transcritical Simple Recuperated cycle. . (a) ∆Tsolar vs ηth diagrams for the Transcritical Simple Recuperated cycle.(b) Solar Share vs ηth diagrams for the Transcritical Simple Recuperated cycle. Fig. 3. ∆Tsolar vs ηth and Solar Share vs ηth diagrams for the Transcritical Simple Recuperated cycle. (a) ηth vs Wsdiagrams for the Supercritical Partial Cooling cycle. (b) ∆Tsolar vs Wsdiagrams for the Supercritical Partial Cooling cycle. Fig. 4. ηth vs Wsand ∆Tsolar vs Wsdiagrams for the Supercritical Partial Cooling cycle. segment of the ∆Tsolar −Wsplots come about because the recuperator outlet temperature increases and eventually achieves the freezing point of FLiNaK when PR is raised. The latter is found at around 454 oC which, considering a safety margin of 20 oC, implies that the minimum salt temperature allowed is in the order of 474 oC. A final remark is that, for given turbine inlet temperature, increasing PR brings about higher ∆Tsolar until FLiNaK achieves the minimum temperature allowed, whereas the effect on ηth is uncertain and can vary depending on the cycle considered; this is observed in Figs. 2(a) and 4(a). This last conclusion might actually become of uttermost importance for the economic feasibility of a CSP plant. Indeed, high ηth leads to a smaller solar field, thus decreasing the plant capital cost, whereas high ∆Tsolar has a direct impact on the size and cost reduction of the TES system. Unfortunately, ηth and ∆Tsolar achieve their maximum values at different PRs and hence a compromise between both figures of merit is mandatory. A deeper analysis is provided in the following sections. 968 Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 6F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 (a) ∆Tsolar vs ηth diagrams for the Supercritical Partial Cooling cycle. (b) Solar Share vs ηth diagrams for the Supercritical Partial Cooling cycle. Fig. 5. ∆Tsolar vs ηth and Solar Share vs ηth diagrams for the Supercritical Partial Cooling cycle. 3. Thermo-economic Analysis The contributions of solar field and thermal energy storage system to the total cost of a 50 MW CSP plant with 10 hours storage capacity (TES made up of hot/cold tanks for extended operation at full load) is now assessed with a dedicated tool based on the open-source software SAM (solar field) [18] and a complementary in-house code (TES) [23] respectively. To this end, a sensitivity analysis of each cost to variations of ηth and ∆Tsolar is done by varying these parameters in the ranges 30-60 % and 90-350 oC respectively. This means that all the cases in the previous section are covered enabling a complete analysis of the contributions of solar field and thermal energy storage system to the total capital cost of the plant. The rated performance of the reference power plant (reference case) in as far as the costs of solar field and storage systems are concerned is defined by ηth =45% and ∆Tsolar =230oC. For this reference case, Fig. 6(a) shows the cost of the solar field; as expected, increasing ηth brings about a significant reduction of this cost. The cost of the storage system presented in Fig. 6(b) includes the cost of purchasing the major equipment as well as those related to installation, insulation, foundations and all the auxiliaries equipment required for system operation. These are corrected with the cost indexes published in the International Journal of Production Economics to account for the time value of money [24], and with exponential cost scaling factors taken from literature [25]. Starting from a detailed project budget reported in [19], this approach provides cost estimates of every single component of a real storage project. Additional information, like wages, labour hours and productivity is taken from [26]. The plots in Fig. 6(b) show that the size of the storage system is inversely proportional to ∆Tsolar. Indeed, when ∆Tsolar increases, the specific energy storage capacity (kJ/kg) of the storage material increases, thus reducing the inventory of molten salts for the same specifications (hours of extended operation at full load). On the other hand, ηth also contributes to reducing the size of the storage system since the heat input required for a given electric output is reduced proportionally to ηth. (a) Solarfield Capital Cost. (b) TES Capital Cost. Fig. 6. Solarfield and TES contributions to Capital Cost, obtained respectively with SAM and in-house code. Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 969 F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 7 These economic assessment becomes more interesting when combined with the thermodynamic analysis presented in the previous section. Let turbine inlet temperature be set to 800oC and turbine inlet pressure be limited to 40 MPa. When PR is varied, the cost of the solar field ranges from 203 to 125 M$ for the Transcritical Simple Recuperated cycle and from 122 to 120 M$ for the Supercritical Partial Cooling layout. Also, the cost of the thermal energy storage system ranges from 120 to 53 M$ for the former cycle and from 68 to 57 M$ for the latter layout. These results come to confirm the earlier comments regarding Figs. 3(a) and 5(a). Whilst the former presents significant variations of the figures of merit, and therefore of the correspondent costs, the latter presents an almost vertical slope, and hence the cost of the solar field is very weakly affected by PR. This is further observed in Fig. 7 where the individual and cumulative costs of solar field and storage system are shown for two different turbine inlet pressures (25 and 40 MPa) and temperatures (800 and 1000oC). Thermal efficiency ηth is also provided on the right vertical axis and the labels in each bar indicate the ∆Tsolar across the solar receiver. The first case (800oC and 25 MPa) stands for the current state of technology whereas the last one (1000oC and 40 MPa) shows the performance attainable by these cycles should significant technology improvements be made. (a) Transcritical Simple Recuperated. (b) Supercritical Partial Cooling. Fig. 7. Capital costs of solarfield and TES for different turbine inlet pressure and temperature (bars). Thermal efficiency (dashed line) ηth and temperature rise across the receiver (labels) ∆Tsolar [oC] also shown. The information in Fig. 7 suggests that raising turbine inlet temperature when pressure is limited to 25 MPa increases efficiency but it does not bring about an economic benefit. This is because despite a slightly lower cost of the solar field, the temperature rise across the receiver drops by more than 100oC due to the higher turbine exhaust temperature. The consequent increase in size and cost of the storage system offsets the aforecited solar field cost reduction. The situation is somewhat similar for the Supercritical Partial Cooling cycle when peak pressure is set to 40 MPa, even if less acute, whilst the impact of a higher temperature in the Transcritical Simple Recuperated system is virtually null. It must be noted though that these statements refer to an economic analysis only since the thermodynamic benefit when TIT is raised is unquestionable (highest ηth). Overall, the largest economic benefit seems to come from pursuing higher operating pressures whilst remaining at moderate turbine inlet temperatures (800oC and 40 MPa). This is because such design conditions yield acceptable efficiency but with a very large ∆Tsolar, the impact of this latter parameter on system cost being dominant for the case considered. 4. Conclusions The first part of the present paper shows a thermodynamic comparison between the Transcritical Simple Recuperated and the Supercritical Partial Cooling cycles for different turbine inlet temperatures and pressures. When pressure is limited to 25 MPa, the first cycle seems to achieve higher specific work Wsand temperature rise across the receiver ∆Tsolar, peaking at 350 kJ/kg and 346 oC respectively, whilst the Supercritical Partial Cooling layout achieves higher ηth, 58%. When the pressure limit is raised to 40 MPa and high solar shares are sought, the Transcritical Simple Recuperated cycle seems to perform slightly better: 56% & 70% for the Supercritical Partial Cooling and 54% & 78% for the Transcritical Simple Recuperated (values corresponding to ηth and SS respectively). 970 Francesco Crespi et al. / Energy Procedia 129 (2017) 963–970 8F. Crespi, D. S´anchez, J.M. Rodr´ıguez, G. Gavagnin /Energy Procedia 00 (2017) 000–000 The second part of the paper presents a thermo-economic analysis to assess how ηth and ∆Tsolar affect the costs of solar field and thermal energy storage system. The results show that the overall costs of these systems are fairly similar (within a 5% margin). Nevertheless, the interesting conclusion from the analysis is the different trend followed by thermal efficiency and costs (of solar field and TES) when turbine inlet temperature and pressure are changed, meaning that a purely thermodynamic optimisation is not advisable. Morover, according to the results, there seems to be a larger benefit coming from higher operating pressures than from increasingly higher temperatures. This conclusion is very positive since higher pressures mean more resistant (bulkier) equipment with essentially the same technology whereas higher temperature would very likely imply new materials and, probably, technologies. As a final comment, the combined thermodynamic and economic analysis presented in this paper makes use of secondary economic figures to highlight that the pathways towards lower capital costs are not as evident as initially thought. This approach makes more sense in sCO2applications due to the fairly complex cycle layouts that are used. References [1] G. Sulzer, Verfahren zur Erzeugung von Arbeit aus Warme, Swiss Patent no269599, 1950. [2] F. Crespi, G. Gavagnin. D. S´ anchez, G.S. Mart´ ınez, Supercritical Carbon Dioxide Cycles for Power Generation: A Review, Applied Energy, 2017, pp . [3] G. 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