The global growth in demand for transportation has been phenomenal, owing to an exponential
increase in population, industrialization and urbanization. This has led to a corresponding increase in
the number of motor vehicles on the roads globally which has made the transport industry one of the
main contributors to environmental pollution and energy insecurity. The profile of alternative fuels
has been rising as an important component of the solutions to the challenge of energy sustainability.
Compressed natural gas is one of the most successful alternative fuels for motor vehicle applications
because of its compatibility with the internal combustion engine, reduced engine maintenance costs,
reduced criteria air pollutants, low cost, abundance and the existence of renewable sourced natural gas
from biomass. The infrastructure for the delivery of compressed natural gas forms part of the primary
energy supply network, which has a significant interdependence with the electricity supply network.
The compressed natural gas fuelling station is one of the vital nodes of the gas delivery network, that
is also reliant on the electricity supply due to the energy intensive compressors that are required to
achieve the right pressure conditions for gas transfer to vehicle tanks.
At the same time, the increase in human population, industrialization, urbanization and market volatility
have threatened the reliability and stability of electricity supply networks. Traditional reliance on
supply upgrading to meet rising demand has proven to be unsustainable due to prohibitively high costs
and associated environmental impact. As a result, demand side management solutions, where better use
of the existing capacity is emphasized have received increasing attention. Demand side management
requires that electricity consumers also play a role in the efficient operation of the electricity grid by
minimizing their electricity usage as well as shifting their flexible loads away from peak electricity
demand periods, so that grid stability is sustained.
In order to participate in demand side management initiatives, operators of compressed natural gas
stations need technically and economically sound strategies for the operation of station compressors
and system components so that energy costs are minimized and gas transfer performance is enhanced.
The compressed natural gas fast-fill station, being the most used configuration for commercial fuelling
service is the focus of the work carried out in this thesis, with a description of solutions to minimize
energy consumption, minimize energy costs and improve gas transfer performance through reduction
of filling time.
For this purpose, firstly, an optimal control strategy that minimizes energy cost by shifting the compressor
load optimally away from the peak electricity pricing period under a time-of-use electricity
tariff, while meeting the gas demand is modelled and evaluated. The controller further minimizes the
switching frequency of the compressor thereby avoiding an increase in wear and tear which would
lead to higher maintenance costs. The results show the effectiveness of the optimal operation model to
achieve a huge reduction in electricity cost for the compressed natural gas station, when compressor-on
time is shifted to offpeak and standard electricity pricing times. Further strategies for the minimization
of switching frequency are compared and the superior approach identified.
Secondly, a hierarchical operation optimization model is designed and evaluated. The strategy achieves
minimized electricity cost and optimal vehicle filling time by optimally controlling the gas dispenser
and priority panel valve function under an optimised schedule of compressor operation. The results
show that the proposed approach is effective in achieving a minimum electricity costs in the upper
layer optimisation while meeting vehicle gas demand over the control horizon. Further, a reduction in
filling time is achieved through a lower layer model predictive control of the pressure-ratio-dependent
fuelling process.
Thirdly, an evaluation of compressor optimal sizing is carried out to minimize energy consumption
and cascade the benefits of optimal operation of the compressed natural gas compressor under the
time-of-use tariff. A comparison of the implication of using a variable speed drive or a fixed speed
drive which are optimally sized is carried out. Results show that indeed further reduction in electricity
costs for the compressed natural gas station is realized when optimally sized compressor drives are
used in combination with optimal operation strategies. Additionally, the four line priority panel is
evaluated for gas transfer performance and found to further increase the efficiency of vehicle fuelling
which is a performance indicator for consumer convenience.
The outcomes of this work demonstrate the effectiveness of the approaches proposed as necessary
to integrate compressed natural gas stations, which are vital nodes of the gas delivery network,
with the demand side management of the electricity grid while at the same time enhancing the gas
transfer performance. This increases the economic efficiency of the compressed natural gas as an
alternative fuel and also advances the goals of demand side management in electricity grid reliability
and stability. / Thesis (PhD)--University of Pretoria, 2019. / Electrical, Electronic and Computer Engineering / PhD / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/77831 |
| Date | January 2019 |
| Creators | Kagiri, Charles Muiruri |
| Contributors | Xia, Xiaohua, u16250304@tuks.co.za, Zhang, Lijun |
| Publisher | University of Pretoria |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2020 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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