Radiative heating of supercritical carbon dioxide flowing through tubes

Brayton power cycles using high temperature, high pressure supercritical carbon dioxide (s-CO2) as the working fluid have been increasingly considered as attractive candidates for solar-thermal power plants. Several configurations of heat exchangers and solar receivers are under investigation, with predicted tube temperatures similar to 1000 K. The inclusion of radiation modeling to capture the effect of absorption bands of s-CO2 and the radiative heat transfer among the equipment surfaces makes the computation costly and time consuming, and is often neglected on the basis of convection being the dominant transport mechanism. In this work, a numerical study has been performed to characterize the heat transfer in simultaneously developing laminar flow of s-CO2 through a circular pipe. The combined effects of convection and radiation are presented by varying the Reynolds number, pipe diameter, length to diameter ratio, wall emissivity and the total wall heat flux. It is shown that neglecting the effects of radiative heat transfer, and in particular the participation of s-CO2 in thermal transport can lead to large errors in predicting wall temperature, and by extension, the component lifetime. The error in wall temperature also leads to erroneous predictions on losses to the environment. The calculations indicate that there is a range of flow conditions over which the design process needs to incorporate radiation modeling. (C) 2016 Elsevier Ltd. All rights reserved

DESIGN REQUIREMENTS FOR DIRECT SUPERCRITICAL CARBON DIOXIDE RECEIVER DEVELOPMENT AND TESTING

This paper establishes the design requirements for the development and testing of direct supercritical carbon dioxide (sCO2) solar receivers. Current design considerations are based on the ASME Boiler and Pressure Vessel Code (BPVC). Section I (BPVC) considers typical boilers/superheaters (i.e. fired pressure vessels) which work under a constant low heat flux. Section VIII (BPVC) considers pressure vessels with operating pressures above 15 psig 2 bar] (i.e. unfired pressure vessels). Section III, Division I - Subsection NH (BPVC) considers a more detailed stress calculation, compared to Section I and Section VIII, and requires a creep-fatigue analysis. The main drawback from using the BPVC exclusively is the large safety requirements developed for nuclear power applications. As a result, a new set of requirements is needed to perform detailed thermal-structural analyses of solar thermal receivers subjected to a spatially-varying, high-intensity heat flux. The last design requirements document of this kind was an interim Sandia report developed in 1979 (SAND79-8183), but it only addresses some of the technical challenges in early-stage steam and molten-salt solar receivers but not the use of sCO2 receivers. This paper presents a combination of the ASME BPVC and ASME B31.1 Code modified appropriately to achieve the reliability requirements in sCO(2) solar power systems. There are five main categories in this requirements document: Operation and Safety, Materials and Manufacturing, Instrumentation, Maintenance and Environmental, and General requirements. This paper also includes the modeling guidelines and input parameters required in computational fluid dynamics and structural analyses utilizing ANSYS Fluent, ANSYS Mechanical, and nCode Design Life. The main purpose of this document is to serve as a reference and guideline for design and testing requirements, as well as to address the technical challenges and provide initial parameters for the computational models that will be employed for the development of sCO(2) receivers

ON-SUN TESTING OF A HIGH TEMPERATURE BLADED SOLAR RECEIVER AND TRANSIENT EFFICIENCY EVALUATION USING AIR

Prior research at Sandia National Laboratories showed the potential advantages of using light-trapping features which are not currently used in direct tubular receivers. A horizontal bladed receiver arrangement showed the best potential for increasing the effective solar absorptance by increasing the ratio of effective surface area to the aperture footprint. Previous test results and models of the bladed receiver showed a receiver efficiency increase over a flat receiver panel of 5-7% over a range of average irradiances, while showing that the receiver tubes can withstand temperatures > 800 degrees C with no issues. The bladed receiver is being tested at various peak heat fluxes ranging 75-150 kW/m(2) under transient conditions using Air as a heat transfer fluid at inlet pressure 250 kPa (-36 psi) using a regulating flow loop. The flow loop was designed and tested to maintain a steady mass flow rate for 15 minutes using pressurized bottles as gas supply. Due to the limited flow -time available, a novel transient methodology to evaluate the thermal efficiencies is presented in this work. Computational fluid dynamics (CFD) models are used to predict the temperature distribution and the resulting transient receiver efficiencies. The CFD simulations results using air as heat transfer fluid have been validated experimentally at the National Solar Thermal Test Facility in Sandia National Labs

COUPLED OPTICAL-THERMAL-FLUID MODELING OF A DIRECTLY HEATED TUBULAR SOLAR RECEIVER FOR SUPERCRITICAL CO2 BRAYTON CYCLE

Recent studies have evaluated closed-loop supercritical carbon dioxide (s-CO2) Brayton cycles to be a higher energy density system in comparison to conventional superheated steam Rankine systems. At turbine inlet conditions of 923K and 25 MPa, high thermal efficiency (similar to 50%) can be achieved. Achieving these high efficiencies will make concentrating solar power (CSP) technologies a competitive alternative to current power generation methods. To incorporate a s-CO2 Brayton power cycle in a solar power tower system, the development of a solar receiver capable of providing an outlet temperature of 923 K (at 25 MPa) is necessary. The s-CO2 will need to increase in temperature by similar to 200 K as it passes through the solar receiver to satisfy the temperature requirements of a s-CO2 Brayton cycle with recuperation and recompression. In this study, an optical-thermal-fluid model was developed to design and evaluate a tubular receiver that will receive a heat input similar to 2 MWth from a heliostat field. The ray-tracing tool SolTrace was used to obtain the heat-flux distribution on the surfaces of the receiver. Computational fluid dynamics (CFD) modeling using the Discrete Ordinates (DO) radiation model was used to predict the temperature distribution and the resulting receiver efficiency. The effect of flow parameters, receiver geometry and radiation absorption by s-CO2 were studied. The receiver surface temperatures were found to be within the safe operational limit while exhibiting a receiver efficiency of similar to 85%

Modeling and analysis of a printed circuit heat exchanger for supercritical CO2 power cycle applications

The supercritical carbon dioxide (S-CO2) based Brayton cycle is a good alternative to conventional power cycles because of high cycle efficiency, compact turbo machinery and compact heat exchangers. In this cycle, the majority of heat transfer (approximately 60-70% of total cycle heat transfer) occurs in the regenerator. For the regenerator, micro-channel heat exchanger is an attractive option because of its high surface-area-to-volume ratio. In this study, the performance of a printed circuit heat exchanger (PCHE) with straight and zigzag channels is evaluated. The study is performed for fully turbulent conditions. The channel diameter and the operating Reynolds number play significant roles in the overall heat transfer and pressure drop of hot and cold channels of S-CO2. For zigzag channels, it is found that a larger bend angle and smaller linear pitch perform better than a smaller bend angle and large linear pitch combination. Correlations for Nusselt number and friction factor are developed using ANSYS Fluent and are subsequently utilized in one dimensional (1D) thermal modeling of the heat exchanger. For the same thermal capacity, the model indicates that the zigzag channel PCHE volume is significantly smaller than that of a straight channel PCHE because of higher heat transfer coefficient. However, the pressure drop incurred in the former design is larger. (C) 2016 Elsevier Ltd. All rights reserved