Optimal approach to energy management and gas delivery of a compressed natural gas station

Abstract

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.