| dc.description.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. | en_US |
