Multi-time period optimized configuration and scheduling of gas storage in gas-fired power plants☆
Introduction
With the growing problems of environmental pollution and energy shortage, the demand to improve the energy efficiency and absorption capacity of renewable energy is increasing. An integrated energy system has become an important trend in the development of the world’s energy sector. The analysis and optimization control of integrated energy systems brings many opportunities and challenges to the research of the energy sector.
On the one hand, the rapid development of CCHP (combined cooling, heating and power) has tightly connected the three kinds of energy systems of cold, heat and electricity systems. Many scholars have conducted a comprehensive study on optimization design and operation of the coupled cooling-heating-power system. In terms of optimization planning, Ref. [1] builds a Multi-temporal simulation model to carry out integrated analysis of electricity-heat-gas distribution networks. Ref. [2] develops a new method to evaluate the thermal contribution of thermal energy storage based on simple procedures which can find the optimal volume both in terms of thermodynamic efficiency and economic profit. Ref. [3] proposes a comprehensive MILP optimization model to optimize the design of a new district. The case studies under four scenarios showed that it can provide the best solution regarding both environmental and economic issues. Ref. [4] presents a novel hybrid CCHP system integrated with compress air energy storage to provide a guiding principle for CCHP system design with the aim of minimizing the total cost and emissions. In terms of optimized operation, an integrated heat and power dispatch model considering the thermal inertia of the district heating network (DHN) [5] and thermal inertia of the building [6] is formulated to exploit the flexibility of district heat system. And an optimal operation model combining the thermal inertia of DHN and buildings is proposed to enhance the absorption of wind power in [7]. In other aspects, Ref. [8] studies the influences of a disturbance in one system on another system through coupling components in the district electricity and heating systems. It suggests that both the fast hydraulic process and slow thermal process should be considered for system security and economic operation. Ref. [9] combines the two cycles of the Stirling cycle and organic Rankine cycle to increase cycle efficiency and obtains the highest efficiency was 41.5%. There is even literature summarizing the optimization research and methods in the coupling cooling-heating-power system [10].
On the other hand, current natural gas power generation has seen dramatic increases. Natural gas has gradually become a significant choice of fuel for the power system for many reasons, such as lower pollutant emissions, higher energy conversion efficiency, a shorter construction period and better load characteristics [11], [12]. It was reported that the generation by natural gas is expected to grow by 230% by 2030 [13]. In China, natural gas is treated as an important part of the energy transformation strategy for China’s energy restructuring and air pollution control. The proportion of natural gas will gradually increase in the primary energy generation. The “Energy Development Strategy Action Plan (2014–2020)” in China clearly states that the proportion of natural gas in primary energy would increase to 10%. With this growing interdependence on natural gas, the operations of the power system are increasingly related to the supply of natural gas. From the systems aspect, there have been several literature analyzing its optimization or security [14], [15], [16], [17]. Ref. [14] proposes a low-carbon economic dispatch model under both constraints of electricity and natural gas systems to deal with the increasing penetration of wind power generation. A probabilistic available transfer capability (ATC) model considering the static security constraints and uncertainties of electricity–gas integrated energy systems is proposed in Ref. [15] to obtain the ATC in the coupled system. Ref. [16] proposes a security-constrained bi-level economic dispatch model for integrated natural gas and electricity systems considering wind power and power-to-gas (P2G) process which aims to minimize the total production cost of electricity and natural gas. A novel quasi-dynamic simulation platform is developed in [17] to analyze the independence and impact of the integration of renewable energy sources into existing electric power systems and natural gas networks. This paper studies from the perspective of gas-fired power plants considering some additional gas storage equipment often be installed in gas power plants to handle possible shortages of the natural gas supply. Ignoring the failure of natural gas systems could result in the disruption of gas transmission. The causes of natural gas shortfalls can be divided into two aspects:
On a long time scale, the natural gas load and power load have seasonal fluctuations. The consumption of natural gas is mainly related to the temperature and weather, showing peaks in winter and troughs in summer. This is because the central heating company consumes a large amount of natural gas for heating in the winter [18], [19]. Power demand shows two peaks in a year, with greater consumption in winter and summer and less consumption in spring and autumn. This is primarily due to the extensive use of electric heaters and air conditioners [20]. Therefore, the seasonal characteristics of natural gas and power loads are sometimes synchronized and sometimes complementary. In the winter, there is a great chance of natural gas shortages when both natural gas and power loads peak at the same time. For example, in the winter of 2015, due to the sustained low temperature, the supply of natural gas in northern China was short on two occasions. To ensure that residential customers had consistent energy, the government took emergency measures to limit the gas flow to industrial customers [21]. Moreover, seasonal gas storage has the opportunity to reduce the annual cost of power generation due to the seasonal price of natural gas [24].
On a short time scale, there are some security constraints in the natural gas network operation. The most important performance measure is the pipeline pressure subject to certain restrictions. If the demand for natural gas and electric power peak simultaneously, the reduction of node pressure will limit the pipeline delivery, which leads to supply shortages for gas plants. This was the case during many failure scenarios in the recent years in the southwest U.S. An extended episode of unusually cold weather caused a high gas demand for residential heating and electrical purposes. To maintain service to residential customers, several gas generating customers were curtailed. Take the blackout in Colorado in 2006 for example. As a result of the gas load peak, the electric utility was forced to shed 85 MW of interruptible and 428 MW of firm loads. More than 323,000 customers were forced to power off for several hours [11], [22].
To ensure and improve the security and reliability of electricity systems, a certain amount of gas storage equipment is expected to be installed in gas-fired power plants to adjust the supply and demand imbalance, especially when gas and electricity load spikes occur simultaneously. There is some current related research on coupled power and natural gas systems that considers gas storage. Ref. [23] noted that the natural gas infrastructure could be utilized in long-term energy storage using P2G (power to gas) technology and large-scale gas storage technology, promoting the development of renewable energy. Ref. [24] proposed a two-stage optimization to assess the gas network operation, including seasonal storage injection/withdrawal, the effects on short-term and long-term gas prices and potential constraints arising in the gas network operation. Ref. [25] studied how distributed natural gas storage smooths out the power demand and improves the operational efficiency in the normal state and during contingencies. The substantial effects of uncertainty in the natural gas supply on the optimal generation schedule and natural gas storage level were stressed in [26]. Ref. [27] proposed a unit commitment model for an electricity-natural gas coupled system considering natural gas storage and the weather effect.
Most of these documents focus on the optimal scheduling of existing gas storage and the impact of gas storage on the power and natural gas coupled system. To the best of the author’s knowledge, there is no literature that proposed a method for assessing the capacity of gas storage facilities in the coupled system. This paper proposed the optimized planning configuration and scheduling model of gas storage in gas power plants for multi-time periods, which can enhance the reliability of power supply in the power system on the premise of ensuring the security of natural gas systems. The contributions of this paper are clarified below.
First, based on the annual load level, considering the restriction of natural gas source supply and seasonal fluctuations of the gas and power loads, a model of gas storage capacity optimization configuration was established using the steady flow equation of a natural gas system and the DC flow equation of a power system. After the configuration of the gas storage according to this model, the gas power plant can not only meet the annual power load level, which improves the reliability of the power supply, but also attain greatly improved annual profit by taking full advantage of the seasonal price of natural gas.
Additionally, to cope with a short-term peak load, considering the restrictions of the natural gas pipeline transmission capacity, this paper proposed an optimized configuration of the gas storage capacity and a day-ahead scheduling model. To reflect the compressibility of natural gas and the inherent gas storage properties of the pipeline, the effect of the line pack was considered in the short-term analytical model. The gas storage facilities play an important role in maintaining the pipeline running pressure and adjusting the peak load. At the same time, some suggestions for the reconstruction of natural gas pipelines were put forward based on the pressure information in the calculation results.
Lastly, considering the uncertainty of power load forecasting, the interval method was used to establish the intraday dispatch gas storage scheduling model. Due to the much slower transmission rate of natural gas than that of electric power, the dynamic hydrodynamic equations of the natural gas system were taken into account. The charge scheme of gas storage obtained by this method can adapt to a certain fluctuating load interval.
The main content is split into 4 sections. Section 1 describes the main motives and content of this research. Section 2 formulates the optimized configuration of the gas storage capacity for a long time scale considering the seasonal fluctuation of the gas load and gas price. Section 3 discusses the impact of the pipeline transmission capacity restrictions on the peak load and proposes an optimized configuration and day ahead scheduling model of gas storage. Section 4 establishes an intraday scheduling model of gas storage adapting to an interval of the load fluctuations. Section 5 presents numerical examples to prove the effectiveness of the three models proposed above, and Section 6 concludes the work.
Section snippets
Optimized configuration of gas storage capacity under seasonal load fluctuations
In consideration of the quarterly fluctuation of natural gas and power loads, a certain amount of gas storage should be provided to supplement the insufficiency of the gas supply for gas-fired power plants in the peak load period. Since the analysed time scale is in months or quarters, the operational constraints of the power system and the natural gas system are both considered as steady-state equations.
Optimized configuration and day-ahead dispatch of gas storage for short-term peak loads considering the line pack
When extreme weather conditions occur, such as sudden snowstorms, the natural gas load and the electrical load peak simultaneously, resulting in a sharp rise in the demand for natural gas in the short term. In this case, even if the gas supply is sufficient in the short term, the load demand may not be met since there is a certain limit on the pipeline transmission capacity and the inlet pressure of the gas turbine. Therefore, the gas power plants need to set a certain gas storage capacity to
Intraday dispatch scheme of gas storage considering the uncertainty of the power load using dynamic model
The optimization results in Section 3 provide a day-ahead gas storage dispatch plan, that is, the initial and final gas storage capacities of each day. However, the load is random in nature, which may be different from the predicted curve. This paper only considers the uncertainty of the power load. Since the power system is always balanced between the generation and demand and the transmission of natural gas takes a much longer time, taking full advantage of the regulation of gas storage to
Numerical studies and discussions
This case consisted an IEEE 30-bus electrical system and a 12-node natural gas system. The structure of natural gas system is shown in Fig. 2.
It is assumed that the IEEE 30-bus system has 6 thermal generators, including 1 gas-fired unit (generator 1) and 5 fossil units (generators 2 ∼ 6). The natural gas system is quoted from Ref. [28] with some modifications. The network parameters are shown in Table 3, Table 4, Table 5, Table 6.
Conclusions
This paper built multi-time period optimized configuration and scheduling of gas storage models for gas power plants. Different constraints, objective functions and gas flow/power models were considered for different time scales. The first model was based on the annual load level and considered the restrictions of the natural gas source supply and seasonal fluctuations of the gas and power load. The case study on this model indicated that despite the fact that installing gas storage devices
Acknowledgment
This work was supported by the National Natural Science Foundation of China (NSFC) (51537006).
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The short version of the paper was presented at ICAE2017, Aug 21–24, Cardiff, UK. This paper is a substantial extension of the short version of the conference paper.