1. Introduction
In the last few years, some technical and environmental aspects have encouraged researchers to utilize the novel high-flexible facilities in power system operations. Gas-fired power units (GFUs) and power-to-gas (P2G) conversion technology are two of those facilities. GFUs are preferred in the integrated electricity and natural gas systems instead of conventional power units (CPUs) due to lower natural gas prices, higher energy transformation performance, and lower air pollutant emission [
1,
2,
3]. Besides this, GFUs are one of the promising solutions to prepare the regulation services for the power systems with the penetration of renewable energy sources (RESs) [
4]. P2G technology is a system that could convert electric power into natural gas and has released energy between electricity and natural gas systems [
5]. This novel technology is called a permanent P2G storage facility, which can accommodate the fluctuations of ever-increasing RESs in power system operations besides preparing regulation services utilizing quickly available electrolysis [
6]. So far, P2G technology has not been provided individually with an economically effective operation in various energy systems, causing high investment and utilization expenditures.
Different technical methodologies are considered to obtain the optimal operation of integrated electricity and natural gas systems and improve the efficiency of electrical energy [
7]. All of these methods are constructed as a framework based on multi-stage optimization programming to optimize the energy flows in the integrated energy systems [
8,
9]. Organizing the secure connection between electricity and natural gas systems establishes several profits for various system sectors, such as utility, customers, and network operators [
10,
11,
12]. To this end, the energy hub (EH) concept has emerged to meet the electrical, thermal, and gas demands in an optimal economic–environmental procedure [
13]. Conversion energy devices are the essential facilities in the EH systems to supply demands in different situations by constituting an optimal connection between several infrastructures. Thus, P2G conversion technology and high-flexible GFU are employed to reduce the total operation cost and increment the energy efficiency [
14,
15]. From these perspectives, more related researches have been accomplished in the investigated literature.
Most of the works that have been investigated fall into the two main categories. The first category is modeling and investigating various bidding strategies of generation companies (GenCos) with different objectives and methodologies. The uncertainties of the electricity price market and outage of GenCos, which impacts profit, are handled in Reference [
16] through the information gap decision theory (IGDT). The uncertainty of day-ahead electricity price is modeled via applying IGDT in Reference [
17], to determine the combined bidding strategy of GenCos and demand response aggregator (DRA). Thus, the obtained optimal results of the proposed problem in Reference [
17] have been confirmed after the realization of market prices. In Reference [
18], a novel teaching–learning-based optimization (TLBO) method was introduced to solve GenCos and major consumers’ bidding strategies in the day-ahead electricity market. In the deregulated electricity market, as covered in Reference [
19], knowing the rival GenCos is helping to gain maximum benefit, which is performed by a grey wolf optimizer (GWO) algorithm. Another meta-heuristic solution called whale optimization algorithm (WOA) was proposed in Reference [
15], to obtain the maximum profit by determining the optimal bidding strategy. Moreover, the authors of Reference [
20] have taken into account the IGDT procedure to distinguish the risk-averse or risk-taker of bidding strategy for price-taker GenCos in the uncertain conditions day-ahead energy and reserve electricity markets. The authors of Reference [
21] presented a robust framework for micro EH (mEH) combined with gas-fired GenCos with analyzing the impacts of integrated demand-response program (IDR) and hydrogen storage system (HSS) technologies. In Reference [
22], a renewable-based power generation multi-objective robust scheduling methodology was presented, to lessen the effects of uncertainties on the proposed system’s stable operation. A bi-level hierarchical decision-making was published in Reference [
23], to specify the character of DRA and GenCos bidding strategy on the adaptability of loads. An upper level of the proposed problem in Reference [
23] is minimizing generation costs, as well as the cost of demand curtailment. Meanwhile, the lower level aims at determining the optimal demand response (DR) quantity and prices of demand curtailment from various aggregators.
The second category of analyzed works is related to the effects of P2G conversion technology on the integrated energy systems. In Reference [
2], a best-coordinated optimal scheduling tool between integrated power and natural gas networks (IPGNs) equipped with a P2G facility was represented. Moreover, a market equilibrium-based game theory model was presented in Reference [
2], to study these effects on the optimal dispatch of IPGNs. The linearized constraints of both electricity and natural gas systems were constructed in Reference [
3], in which the impacts of deploying P2G facilities on daily economic scheduling were investigated. Moreover, Reference [
24] focused on the risk-averse approach, which was modeled as an improved conditional value at risk (CVaR) to handle the uncertainty associated with wind and solar power units. A two-stage robust scheduling method was introduced in Reference [
25] for the IPGNs equipped with P2G and hydrogen compressed natural gas (HCNG) technologies, to decrease operational risk and increase the system’s stability in the worst cases. In Reference [
26], a two-stage multi-objective stochastic unit-commitment approach was proposed for IPGNs, in which flexible energy devices as P2G facility and DR are embedded to lessen the environmental gas emissions and operating costs. The optimal scheduling structure of an EH was reported in Reference [
27]; it was equipped with GenCos and multi-carrier energy storage systems, i.e., a P2G system, in order to demonstrate the effectiveness of the P2G system on the operation costs. In Reference [
15], the valuable outcomes of the P2G facility and compressed air energy storage (CAES) system on reducing the RESs’ intermittency and operation costs of the proposed EH were indicated. In addition to the RESs’ uncertainty, the fluctuations of the electricity prices and various demands were considered in Reference [
15], where CVaR was utilized to analyze the risk of the introduced strategy. A probabilistic optimal scheduling framework of a viable P2G-based IPGNs was presented in Reference [
28], in which load shifting-based DR programs were taken into account. In Reference [
29], a probabilistic power flow methodology was established for IPGNs coupled with the P2G system and wind power, and it indicated that the P2G system decreases the impacts of wind power on the security of the power system. In Reference [
14], a hybrid bi-level IGDT-stochastic co-optimization framework for IPGNs was proposed, in which gas demand, electrical demand, and wind-power generation were considered as uncertain parameters.
All of the related research works we investigated in the literature review are associated with the optimal bidding strategies of GenCos and the impacts of the P2G facility on the integrated electricity and natural gas systems, which are proposed with various objectives and solution approaches. However, all of those researches are mainly about the optimal scheduling and bidding frameworks without extra wind-power utilization. Thus, according to this issue, there is no focus on the P2G conversion facility’s bidding strategy in the integrated electricity and natural gas networks. This topic is the research gap of the analyzed works. To this end, in this paper, a coordinated optimal bidding strategy of hybrid energy system coupled with GenCos and P2G conversion technology and also with wind power unit (WPU) is proposed to participate in the day-ahead energy market. Applying the P2G facility in the combined manner has a significant impact on producing electric power of GFUs with considering gas-consumption limitations in order to participate in the energy market. In addition to obtaining a realistic and accurate optimal bidding strategy, the electricity market price’s uncertainty is handled via the robust optimization technique. A review of the exitance work portfolio is provided in
Table 1. Briefly, the main contributions of this paper are summarized as follows:
Introducing a large-scale P2G conversion technology to handle the gas-consumption limitation of GFUs.
A risk-based method is considered to handle the day-ahead electricity-market-price uncertainty in the scheduling problem.
An optimal bidding strategy for a hybrid energy system coupled with GFU-P2G-WPU facilities is proposed to participate in the day-ahead energy market.
The rest of this paper is organized as follows.
Section 2 describes the mechanism and energy contribution of permanent energy storage, i.e., a P2G conversion facility. The proposed problem formulation is indicated in
Section 3. The numerical results and analysis of two case studies are introduced in
Section 4, to represent the capability and effectiveness of the considered optimal bidding strategy for a hybrid energy system. Finally,
Section 5 concludes and reports noticeable outcomes.
7. Conclusions
In this paper, a robust optimal operation strategy of a hybrid energy system, consisting of a gas-fired unit (GFU), power-to-gas (P2G) facility, and wind power unit (WPU) for the day-ahead energy market, was proposed. The proposed energy system has limitations in fuel purchasing and selling power. The wind power is still non-dispatched because of transmission-line limitation, and also the GFU cannot reach its maximum capacity during high-electricity-price periods, due to fuel limitation. The proposed P2G technology assists the energy system in solving these issues. During high-wind-penetration hours, the excess of wind power that cannot be exported converts to gas and is stored at the gas storage system (GSS). Then, during high-electricity-price hours, GSS discharges to supply the GFU. In doing so, the profit of the presented hybrid energy system increases by nearly 1%. As indicated in numerical results, the contribution of P2G conversion technology is marked as backup facilities in providing required natural gas for GFUs, which have limitations in gas fuel consumption. It can also be stated that the curtailed and non-dispatched power of WPU is utilized as consumed power by the P2G facility, to contribute GFUs more actively in proposing their bids into the energy market. Finally, to investigate the market price uncertainty in obtaining maximum profit, robust optimization was performed to manage the uncertainty. The presented robust model is based on the uncertainty budget, and the amount of risk can be controlled. Increasing the risk by adjusting a higher uncertainty budget causes a decrease in electricity price and, consequently, obtained profit. Considering the price uncertainty for the whole time horizon, i.e., 24 h, it was reduced by about 22% in the profit. As a result, getting more robustness levels will lead to a lower profit of the system. Furthermore, the current research can be extended by considering the upstream power grid and natural gas network constraints with more details. In addition, the impact of P2G technology can be investigated in different multiple-energy-carrier systems, such as energy hubs, multi-carrier microgrids, etc. Moreover, other associated uncertainties can be taken into account, and their effect can be evaluated.