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968 B.D. Iverson et al. / Applied Energy 111 (2013) 957–970 selection in high temperature receiver materials at a minimum for the next generation of solar power plants. 4.5. Impact on cost goals Comprehensive, critically reviewed costing data for the major components of a commercial sized (10MWe) recompression, closed-Brayton cycle for CSP applications were not found in the open literature. Private parties interested in developing compo- nents consider costing data proprietary. To fill this cost informa- tion void, Sandia National Laboratories, Oak Ridge National Laboratory (ORNL), and the Department of Energy (Office of Nucle- ar Energy) are initiating research into these various costs. The re- sult of this effort is intended to be a modeling tool that predicts the levelized cost of electricity (LCOE) for these systems and incor- porate the information into the current LCOE program that ORNL maintains. As the working fluid operates in the supercritical phase, the turbomachinery size [35] and cost may be lower but extensive recuperation will necessitate costly heat exchange. Assuming similar power block costs as that for steam-Rankine, the estimated impact of the proposed Brayton cycle to achieve SunShot goals is assessed using the National Renewable Energy Laboratory’s System Advisor Model (SAM, https://sam.nrel.gov/). Assuming a molten salt power-tower plant model and adjusting the power block efficiency (54% gross) and receiver temperature (700 °C salt) to account for an indirect, dry-cooled, sCO2 Brayton cycle, the LCOE for a 100 MWe system has been estimated. In order to achieve 6 ¢/kW h (real), a solar multiple of 3.1 and 16 h of stor- age is required for the following SunShot-driven system: Cost assumptions: site preparation = 10 [$/m2], solar field = 75 [$/m2], power plant = 1,160 [$/kW], tower/receiver = 150 [$/ kWt], thermal storage = 15 [$/kWt], contingency = 0%, indirect (sales tax and land) = 17.8%, interest during construction = 6.0%, O&M = 40 [$/kW yr]. SunShot financial assumptions: discount rate = 5.5%, inflation rate = 3%, debt rate = 6%, state income tax = 5%, return on equity = 15%, debt fraction = 62%, federal tax = 35%, deprecia- tion = 5 yr MACRS, ITC = 0%. Of particular note are the large solar multiple and 16 h of stor- age required to achieve the desired SunShot LCOE target. This re- sults in a capacity factor of 73.1% and is in the range that a direct CO2 receiver/system cannot efficiently provide (see Fig. 7). Capac- ity factors of this magnitude must use the storage media in the re- ceiver, thereby requiring liquid-to-sCO2 heat exchangers of the type pursued in this proposal for implementing sCO2 Brayton to capitalize on the cycle efficiency benefits. 5. Conclusions The sCO2 Brayton cycle has been shown to have significant effi- ciency benefits especially as solar-thermal power plants increase their operating temperatures above 600 °C. In particular, part heat load operation, common to a solar resource, appears manageable especially for short durations (e.g. short-term cloud cover) due to thermal capacitance in the system and piping. Therefore it is rec- sented in this work and would approach approximately 24% with minor modifications to improve insulation. Predicted efficiencies still far short of the 50% thermal efficien- cies claimed for sCO2 at a 600 °C turbine inlet temperature. This is primarily a limitation of the laboratory-scale demonstration tur- bine used for the present study. At around 250 kWe in size, the test facility was intended to be large enough to confront the fundamen- tal issues for sCO2 Brayton cycle technology, but small enough to be affordable over several years of incremental funding. For adop- tion of sCO2 in a solar-thermal power plant, a number of required advances remain and are largely addressed by moving to larger equipment in 10 MWe range. For large capacity factors and indirect systems, heat exchange between CO2 and a secondary fluid amena- ble to solar is also required. This represents a significant challenge in terms of material selection as well as heat exchanger design. Improvements in bearings and seals to prevent leakage are also re- quired, especially as the system scales up from the small prototype included here. Acknowledgements This manuscript has been authored by Sandia National Labora- tories, a multi-program laboratory managed and operated by San- dia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. The authors would also like to thank Craig Turchi for assistance with the economic analysis. References [1] Hu E, Yang Y, Nishimura A, Yilmaz F, Kouzani A. Solar thermal aided power generation. Appl Energy 2010;87:2881–5. [2] Denholm P, Ela E, Kirby B, Milligan M. The role of energy storage with renewable electricity generation. NREL/TP-6A2-47187, January 2010, National Renewable Energy Laboratory; 2010. [3] Denholm P, Hand M. Grid flexibility and storage required to achieve very high penetration of variable renewable electricity. Energy Policy 2011;39:1817–30. [4] Sioshansi R, Denholm P. The value of concentrating solar power and thermal energy storage. NREL-TP-6A2-45833, February 2010, National Renewable Energy Laboratory; 2010. [5] Kolb GJ, Ho CK, Mancini TR, Gary JA. Power tower technology roadmap cost reduction plan. SAND2011-2419, April 2011, Sandia National Laboratories, Albuquerque, NM; 2011. [6] U.S. Department of Energy, SunShot Vision Study; 2012.PDF Image | Supercritical CO2 Brayton cycles for solar-thermal energy
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