Abstract. The usage of molten nitrate salt as heat transfer fluid and thermal storage medium decouples the generation of electricity from the variable nature of the solar resource, allowing CSP plants to avoid curtailment and match production with demand. This however brings some unique challenges for the design of the molten salt central receiver (MSCR). An aspect critical to the use of molten nitrate (60wt%/40wt% -NaNO 3 /KNO 3 ) salt as heat transfer fluid in the MSCR is to understand its heat transfer behavior. Alstom collaborated with the University of Wisconsin to conduct a series of experiments and experimentally determined the heat transfer coefficients of molten nitrate salt up to high Reynolds number (Re > 2.0E5) and heat flux (q″ > 1000 kW/m 2 ), conditions heretofore not reported in the literature. A cartridge heater instrumented with thermocouples was installed inside a stainless steel pipe to form an annular test section. The test section was installed in the molten salt flow loop at the University of Wisconsin facility, and operated over a range of test conditions to determine heat transfer data that covered the expected operating regime of a practical molten salt receiver. Heat transfer data were compared to widely accepted correlations found in heat transfer literature, including that of Gnielinski. At lower Reynolds number conditions, the results from this work concurred with the molten salt heat transfer data reported in literature and followed the aforementioned correlations. However, in the region of interest for practical receiver design, the correlations did not accurately model the experimentally determined heat transfer data. Two major effects were observed: (i) all other factors remaining constant, the Nusselt numbers gradually plateaued at higher Reynolds number; and (ii) at higher Reynolds number a positive interaction of heat flux on Nusselt number was noted. These effects are definitely not modeled by the existing correlations. In this paper a new Nusselt number correlation is shown to accurately model the experimental data. A theory to explain the heat transfer trends at higher Reynolds number and heat flux is proposed. Finally, the impact of these results on receiver design is discussed, with comparison between correlations available from literature and that proposed in this study.

ALSTOM is developing and testing a new and more efficient coal combustion technology, including a new type of steam generator known as a “circulating moving bed (CMBTM) combustion system combustor.” The CMBTM combustion system technology involves a novel method of solid fuel combustion and heat transfer. In this design, a heat exchanger will heat the energy cycle working fluid, steam or air, to the high temperature levels required for advanced power generation systems. This will produce a step change in both performance and capital costs relative to today’s pulverized coal and fluid bed boiler designs. In addition to high temperature Rankine cycles, the CMBTM combustion system is an enabling technology for hydrogen production and CO2 capture from combustion systems utilizing innovative chemical looping airblown gasification and syngas decarbonization. ALSTOM’s 3MWth Multi-Use Combustion Test Facility has been modified to allow operation in CMBTM combustion system mode. This paper summarizes the results of this program, which includes performance results from pilot plant testing. Participants include the U.S. DOE, ALSTOM, the University of Massachusetts, and the Massachusetts Institute of Technology. The total program cost is $2,485,468 with the DOE’s National Energy Technology Laboratory (NETL) providing 60% of the funding under Cooperative Agreement No. DE-FC26-01NT41223.