Parabolic trough systems are one of the most commercially and technically developed
technologies for concentrated solar power. With the current research and development
efforts, the cost of electricity from these systems is approaching the cost of electricity from
medium-sized coal-fired power plants. Some of the cost-cutting options for parabolic trough
systems include: (i) increasing the sizes of the concentrators to improve the system’s
concentration ratio and to reduce the number of drives and controls and (ii) improving the
system’s optical efficiency. However, the increase in the concentration ratios of these systems
requires improved performance of receiver tubes to minimise the absorber tube
circumferential temperature difference, receiver thermal loss and entropy generation rates in
the receiver. As such, the prediction of the absorber tube’s circumferential temperature
difference, receiver thermal performance and entropy generation rates in parabolic trough
receivers therefore, becomes very important as concentration ratios increase. In this study, the thermal and thermodynamic performance of parabolic trough receivers at
different Reynolds numbers, inlet temperatures and rim angles as concentration ratios
increase are investigated. The potential for improved receiver thermal and thermodynamic
performance with heat transfer enhancement using wall-detached twisted tape inserts,
perforated plate inserts and perforated conical inserts is also evaluated.
In this work, the heat transfer, fluid flow and thermodynamic performance of a parabolic
trough receiver were analysed numerically by solving the governing equations using a
general purpose computational fluid dynamics code. SolTrace, an optical modelling tool that
uses Monte-Carlo ray tracing techniques was used to obtain the heat flux profiles on the
receiver’s absorber tube. These heat flux profiles were then coupled to the CFD code by
means of user-defined functions for the subsequent analysis of the thermal and
thermodynamic performance of the receiver. With this approach, actual non-uniform heat
flux profiles and actual non-uniform temperature distribution in the receiver different from
constant heat flux profiles and constant temperature distribution often used in other studies
were obtained.
Both thermodynamic and multi-objective optimisation approaches were used to obtain
optimal configurations of the proposed heat transfer enhancement techniques. For
thermodynamic optimisation, the entropy generation minimisation method was used.
Whereas, the multi-objective optimisation approach was implemented in ANSYS
DesignXplorer to obtain Pareto solutions for maximum heat transfer and minimum fluid
friction for each of the heat transfer enhancement techniques.
Results showed that rim angles lower than 60o gave high absorber tube circumferential
temperature differences, higher receiver thermal loss and higher entropy generation rates,
especially for flow rates lower than 43 m3/h. The entropy generation rates reduced as the inlet
temperature increased, increased as the rim angles reduced and as concentration ratios
increased. Existence of an optimal Reynolds number at which entropy generation is a
minimum for any given inlet temperature, rim angle and concentration ratio is demonstrated.
In addition, for the heat transfer enhancement techniques considered, correlations for the
Nusselt number and fluid friction were obtained and presented. With heat transfer
enhancement, the thermal efficiency of the receiver increased in the range 5% – 10%, 3% – 8% and 1.2% – 8% with twisted tape inserts, perforated conical inserts and perforated
plate inserts respectively. Results also show that with heat transfer enhancement, the absorber
tube’s circumferential temperature differences reduce in the range 4% – 68%, 3.4 – 56% and
up to 67% with twisted tape inserts, perforated conical inserts and perforated plate inserts
respectively. Furthermore, the entropy generation rates were reduced by up to 59%, 45% and
53% with twisted tape inserts, perforated conical inserts and perforated plate inserts
respectively. Moreover, using multi-objective optimisation, Pareto optimal solutions were
obtained and presented for each heat transfer enhancement technique.
In summary, results from this study demonstrate that for a parabolic trough system, rim
angles, concentration ratios, flow rates and inlet temperatures have a strong influence on the
thermal and thermodynamic performance of the parabolic trough receiver. The potential for
improved receiver thermal and thermodynamic performance with heat transfer enhancement
has also been demonstrated. Overall, this study provides useful knowledge for improved
design and efficient operation of parabolic trough systems. / Thesis (PhD)--University of Pretoria, 2015. / tm2015 / Mechanical and Aeronautical Engineering / PhD / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/45963 |
| Date | January 2015 |
| Creators | Mwesigye, Aggrey |
| Contributors | Bello-Ochende, Tunde, Meyer, Josua P. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2015 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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