Development of Tubular Solar Receiver with Supercritical Carbon Dioxide as Working Fluid

Abstract

Over the past decade, solar powered supercritical carbon dioxide (s-CO2) based Brayton cycle has been identified as a promising candidate due to its potentially high cycle efficiency (50%, for turbine inlet temperatures of ~ 1000 K). Materialization of this cycle requires development of solar receivers capable of heating s-CO2 by over 200 K, to a receiver outlet temperature of about 1000 K. Due to the extreme outlet conditions (~1000 K, 20 MPa), tubular solar receivers which typically employ panels consisting of metallic circular tubes for transfer of the incident concentrated solar radiation to the fluid flowing within the tubes are considered to be a suitable option for direct heating of s-CO2. In the design of receivers/heat exchangers for s-CO2 Brayton cycle, equipment wall temperatures above 1000 K are anticipated. While CO2 is considered to be transparent to the solar radiation spectrum, it has considerable absorption component in the longer wavelength range. This absorption effect may be present for s-CO2 also, yet its participating nature in radiation heat transfer has been traditionally ignored for flow through tubes. In this study, a numerical analysis using existing analytical data for s-CO2 absorption spectrum has been performed to study the fundamental aspects of a developing laminar flow of s-CO2 for a constant heat flux boundary condition. It is observed that while the velocity profiles remain largely unaffected, augmentation of overall heat transfer coefficient and Nusselt number due to presence of radiation heat transfer in addition to convection and conduction has a significant effect on the temperature distribution on the tube wall and its vicinity. It is found that for accurate design and estimation of heat transfer performance of s-CO2 equipment, the participating nature of s-CO2 can be critical for laminar and low Reynolds number turbulent flows. In general, the effect of absorption can be increasingly significant for lower values of Reynolds number and larger values of tube internal emissivity, tube diameter, tube length and the incident heat flux. In addition to absorption of radiation, emission by s-CO2 may also be significant and has been ignored in the literature in spite of the high temperatures involved. To investigate this aspect, a novel experimental method for measurement of radiation emitted by s-CO2 at high pressure and high temperature is presented in this thesis. Due to high pressure conditions, use of conventional spectroscopic methods to measure radiative properties of s-CO2 is challenging. In the present method, supercritical conditions are created in a shock tube by using carbon dioxide (CO2) as the driven gas, and a platinum thin film sensor is used to measure the radiation heat flux emitted by s-CO2. The total emissivity for s-CO2 is estimated and the value compares favourably with that predicted theoretically using a standard method available in literature. It is estimated that the total emissivity value in supercritical conditions is nearly 0.2 for the conditions studied, implying that s-CO2 acts as a participating medium for radiation heat transfer. The outcome of this study has a significant impact on the design and analysis of heat transfer equipment where laminar or low Reynolds number turbulent flows are encountered. For accurate and realistic design of a tubular solar receiver, a novel methodology for coupled optical-thermal-fluid analysis is presented in this work and the proposed methodology is utilized for developing a prototype of s-CO2 receiver consisting of panels constituted by tubes. First, a preliminary analysis is presented, detailing the methodology for coupled analysis. The effect of staggering the tubes to increase the effective absorptance and reduce the reflective losses is explored. A receiver consisting of a single large panel is analysed to establish the methodology and to estimate the tube wall temperature and efficiency for a typical incident flux distribution on the receiver tubes in conjunction with flow of s-CO2 through the tubes. Subsequently, detailed optical-thermal-fluid analysis and design of a s-CO2 tubular receiver with flat panels is performed. Different flow arrangements with and without recirculation, aim point strategies and power levels of operation are studied for comprehensive evaluation of the receiver performance under different conditions. It is found that the receiver designed is able to provide the required temperature rise while the pressure drop and peak receiver temperatures are within allowable limits. As found in a recent study at Sandia National Laboratories, arrangement of the receiver panels in the form of blades can result in an increase in the overall receiver efficiency by up to 5 % compared to the flat receiver arrangement, due to better optics. This bladed receiver arrangement is adopted in the final stage of this work for modelling, testing and design validation of the s-CO2 receiver using compressed air as the heat transfer fluid. Details of the bladed receiver configuration, coupled modelling, prototyping and testing are presented in this thesis. For high temperature on-sun testing on a solar tower, despite the limited availability of pressurized air, a unique test strategy is employed. The receiver is successfully demonstrated to safely heat air up to a temperature of 700 K, with receiver wall temperatures approaching 1000 K. To account for the thermal mass associated with the transient nature of the tests and heating of the non-irradiated part of the receiver structure during the on-sun tests, a modified receiver efficiency calculation is proposed in this work. The agreement between the measured and simulated values of receiver efficiency, temperature and heat flux distributions is found to be satisfactory.