Dual Loop line-focusing solar power plants with supercritical Brayton power cycles

Abstract This study is focused on proposing the combination of a Dual Loop solar field, with Dowtherm A and the Solar Salt as heat transfer fluids in parabolic or linear Fresnel solar collectors, coupled to supercritical Carbon Dioxide (s-CO2) Brayton power cycle. The Dual-Loop justification relies on gaining the synergies provided by the different heat transfer fluids properties. The oils advantages are related with the operating experience accumulated in numerous solar power plants deployed around the World, assuring the commercial equipment availability. Also the pipes metal corrosion with oil is much lower than with molten salt. The pipes material cost saving is significant with the oil alternative. The thermal oil main constraint is imposed by the maximum operating temperature (around 400 °C) for avoiding chemical decomposition and degradation, stablishing the plant threshold efficiency 37% due to Carnot principle. On the other hand the Solar Salt mixture (60%NaNO3 40%KNO3) maximum operating temperature goes up to 550 °C, but the freezing point is stablished around 220 °C requiring pipes and equipment electrical heating for avoiding salts solidification at low temperature. Regarding the balance of plant, the s-CO2 power cycle is the most promising alternative to the actual Rankine power cycle for increasing the plant energy efficiency, reducing the solar collector aperture area and minimizing the equipment dimensions and civil work. Three Brayton cycles configurations with reheating were assessed integrated with the line-focusing Dual-Loop solar field: the simple Brayton cycle (SB), the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling (RCMCI). The power cycle operating thermodynamic parameters (split flow, reheating pressure, mass flow and pressure ratio) were optimized with unconstrained multivariable algorithms: SUBPLEX, UOBYQA and NEWUOA. The main conclusion deducted is the significant efficiency improvement when adopting the s-CO2 Brayton cycle in comparison with the Rankine legacy solution. The Dual-Loop solar field integrated with a Rankine cycle provides a gross efficiency around 41.8%, but when coupling to s-CO2 Brayton RC or RCMCI the plant efficiency goes up to ≈50%. It was also demonstrated the beneficial effect of increasing the total heat exchangers (recuperators) conductance (UA) for optimizing the Brayton cycles efficiency and minimizing the solar field aperture area for a fixed power output, only limited by the minimum pinch point temperature in heat exchangers.

[1]  M. J. D. Powell,et al.  UOBYQA: unconstrained optimization by quadratic approximation , 2002, Math. Program..

[2]  M. McLinden,et al.  NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, Version 8.0 , 2007 .

[3]  Guillermo San Miguel,et al.  Technical and Environmental Analysis of Parabolic Trough Concentrating Solar Power (CSP) Technologies , 2015 .

[4]  M. A. Reyes-Belmonte,et al.  Optimization of a recompression supercritical carbon dioxide cycle for an innovative central receiver solar power plant , 2016 .

[5]  Hongguang Jin,et al.  A three-dimensional simulation of a parabolic trough solar collector system using molten salt as heat transfer fluid , 2014 .

[6]  M. Driscoll,et al.  The Supercritical Carbon Dioxide Power Cycle: Comparison to Other Advanced Power Cycles , 2006 .

[7]  Vittorio Ferraro,et al.  On the performance of CSP oil-cooled plants, with and without heat storage in tanks of molten salts , 2015 .

[8]  R. Forristall,et al.  Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in Engineering Equation Solver , 2003 .

[9]  Hongguang Jin,et al.  Thermodynamics investigation of a solar power system integrated oil and molten salt as heat transfer fluids , 2016 .

[10]  Vaclav Dostal,et al.  A supercritical carbon dioxide cycle for next generation nuclear reactors , 2004 .

[11]  John J. Dyreby.  Modeling the Supercritical Carbon Dioxide Brayton Cycle with Recompression , 2014 .

[12]  T. Rowan.  Functional stability analysis of numerical algorithms , 1990 .

[13]  Robbie McNaughton,et al.  Effect of Pressure Drop and Reheating on Thermal and Exergetic Performance of Supercritical Carbon Dioxide Brayton Cycles Integrated With a Solar Central Receiver , 2015 .

[14]  Fritz Zaversky,et al.  Object-oriented modeling for the transient performance simulation of parabolic trough collectors using molten salt as heat transfer fluid , 2013 .

[15]  M. Powell.  The NEWUOA software for unconstrained optimization without derivatives , 2006 .

[16]  G. Angelino.  Carbon Dioxide Condensation Cycles For Power Production , 1968 .

[17]  G. Luna,et al.  Archimede Solar Energy Molten Salt Parabolic Trough Demo Plant: A Step Ahead towards the New Frontiers of CSP , 2015 .

[18]  D. Kearney,et al.  Assessment of a Molten Salt Heat Transfer Fluid in a Parabolic Trough Solar Field , 2003 .

[19]  SOLAR THERMAL ENERGY PRODUCTION: GUIDELINES AND FUTURE PROGRAMMES OF ENEA , 2007 .

[20]  F. Matino,et al.  Molten Salt Receivers Operated on Parabolic Trough Demo Plant and in Laboratory Conditions , 2015 .

[21]  G. Angelino.  Real Gas Effects in Carbon Dioxide Cycles , 1969 .

[22]  G. Morin,et al.  Molten Salt as a Heat Transfer Fluid in a Linear Fresnel Collector – Commercial Application Backed by Demonstration , 2015 .

[23]  G. Angelino.  Perspectives for the Liquid Phase Compression Gas Turbine , 1967 .

[24]  J. M. Coulson,et al.  Heat Transfer , 2018, Finite Element Method for Solids and Structures.

[25]  E. Feher.  SUPERCRITICAL THERMODYNAMIC POWER CYCLE. , 1967 .

[26]  A. K. Vogel,et al.  High Temperatures in Line Focusing Systems: Dual Loop Cycle Efficiency and Heat Losses☆ , 2015 .

[27]  Seungjoon Baik,et al.  Review of supercritical CO2 power cycle technology and current status of research and development , 2015 .

[28]  Augusto Maccari,et al.  Archimede solar energy molten salt parabolic trough demo plant: Improvements and second year of operation , 2016 .

[29]  C. Turchi,et al.  Thermodynamic Study of Advanced Supercritical Carbon Dioxide Power Cycles for Concentrating Solar Power Systems , 2013 .

[30]  Ali Abbas,et al.  A comparative study of solar heliostat assisted supercritical CO2 recompression Brayton cycles: Dynamic modelling and control strategies , 2017 .

[31]  Luca Rinaldi,et al.  Effects assessment of 10 functioning years on the main components of the molten salt PCS experimental facility of ENEA , 2016 .

[32]  Fahad A. Al-Sulaiman,et al.  Energy and exergy analyses of solar tower power plant driven supercritical carbon dioxide recompression cycles for six different locations , 2017 .

[33]  Justin Raade,et al.  Development of Molten Salt Heat Transfer Fluid With Low Melting Point and High Thermal Stability , 2011 .

[34]  Ennio Macchi,et al.  Comparison of different solar plants based on parabolic trough technology , 2012 .

[35]  C. Turchi,et al.  A Comparison of Supercritical Carbon Dioxide Power Cycle Configurations with an Emphasis on CSP Applications , 2013 .