1. Introduction
Recently, renewable energy sources (RESs) have been introduced to the power system of several countries, supported by political investment. However, fluctuation in wholesale electricity prices has increased because of the intermittent nature of generation from RESs [
1]. By contrast, the retail market generally offers flat pricing or block pricing, because customers cannot continuously respond to price fluctuations [
2]. Therefore, retailers who secure energy in the wholesale market to serve the demand of customers face price risks caused by the difference between wholesale prices and retail rates. This is a challenge for retailers trying to maximize profits [
3].
To hedge the price risk, retailers can undertake several strategies such as power purchase agreements, optimizing self-generation portfolio, and financial contracts on the secondary market. Among the hedging methods, implementing an electricity sales plan in the retail market is generally the adopted method in restructured electricity markets [
4]. This can be accomplished through the optimal operation of a demand response program, which is divided into two categories: Incentive-based programs (IBPs) and price-based programs (PBPs) [
5,
6]. Using IBPs, retailers provide incentive payments to customers depending on the amount of reduced load. To measure demand reduction, retailers should determine the customer baseline load (CBL), which varies significantly depending on the evaluation method. Therefore, a fundamental weakness of the IBP is that there is no reliable baseline [
7]. By contrast, the PBP is based on dynamic pricing, with higher rates during peak periods than off-peak periods. Customers participating in a PBP voluntarily reduce their power consumption in response to time-varying price signals. Pricing methods are typically divided into three types: Real-time pricing (RTP), time-of-use (TOU) pricing, and critical peak pricing (CPP) [
5]. RTP is the most economically efficient pricing to hedge against price risk because retail rates can directly reflect the variations in wholesale prices [
7]. However, customers find it difficult to participate in RTP because of continuously changing rates. Compared to RTP, TOU is easily accepted by customers because it has a simple rate structure, which divides a day into several blocks and charges a constant rate in each block. With TOU, however, it is difficult to induce customers to reduce demand during critical peak periods [
8]. By contrast, CPP has several advantages over the other two pricing models. For example, CPP can be easily implemented because it is based on the rate structure of TOU [
9]. Likewise, it can enhance a customer’s price responsiveness by applying an extremely high rate during critical peak periods [
10].
Although several studies analyze the responses of various demand types under CPP with experimental data [
11,
12,
13,
14,
15,
16], only a few have investigated the implementation of CPP. In Joo et al. [
17], a retailer decides, using a method similar to a swing option problem, when to call a critical event to maximize profit. Park et al. [
18] proposed guidelines for designing various parameters in CPP, such as optimal peak rates, event duration, and number of events. In addition, the optimal schedule of critical events was determined. Park et al. [
19] investigated the impact of the payback phenomenon on the optimal schedule of critical events and the optimal peak rate. Zhang et al. [
20] proposed a decision model to determine critical events by considering the interests of both customers and retailers. This model is deterministically re-calculated daily based on day-ahead price updates for the remaining scheduling horizon. In Chen et al. [
21], dynamic thresholds to trigger critical events are computed daily through dynamic programming based on probability distributions of temperature and price. Moreover, Zhang [
22] used a stochastic approach to decide the optimal schedule of critical events by formulating an objective function as a mixed integer non-linear programming (MINLP) problem. However, previous studies have not fully analyzed the impact of balancing cost on the CPP operation.
By contrast, in most power markets, the balancing cost is allocated not only to the generation company, but also to the retailer to motivate each to reduce their own variability and uncertainty [
23]. The power exchange for frequency control (PXFC) market first proposed in [
24] has been investigated in several studies to evaluate the optimal strategy of market participants with balancing cost [
23,
25,
26]. In a PXFC market, the balancing cost is allocated to each retailer based on a cost-causality principle in the form of a band capacity purchase [
25]. Therefore, we assume the PXFC market circumstance to consider the impact of the balancing cost on strategy of retailer.
In this study, we propose an optimal strategy for a retailer who owns a self-generation facility to maximize its expected profit. The problem is formulated to optimize both the bidding schedule in the PXFC market and the CPP operation in the retail market. Unlike previous works, we introduce a multi-stage stochastic problem (MSSP), incorporating the retailer’s uncertainties such as electricity price, demand, and photovoltaic (PV) generation, represented by multiple scenarios. In contrast to a mean-value deterministic stochastic model, MSSP can make decisions sequentially with the uncertainties revealed over periods. Thus, successive decisions can be modified through future observation of uncertainties. Based on the rolling horizon (RH) method, we identify the effectiveness of the proposed MSSP model compared with that of a mean-value deterministic model. Moreover, the MINLP is reformulated as a mixed integer linear problem (MILP) to ensure a global optimum.
The contributions and novelty of the proposed method can be summarized as follows: (1) We suggest an optimization problem to determine the bidding strategy and schedule critical events to maximize the expected profit of retailers considering balancing costs; (2) we develop an MSSP for decisions to be taken recursively with the realization of uncertainties; (3) we linearize the non-linear problem with a binary variable; (4) we analyze the impact of various parameters on the retailer’s optimal strategy and profit.
The rest of the paper is organized as follows.
Section 2 introduces the background of this study, including the PXFC market, CPP, and MSSP based on the RH method.
Section 3 presents the mathematical formulation of MSSP and the proposed algorithm based on the RH method.
Section 4 provides a numerical simulation and discusses the results obtained from various case studies. Finally,
Section 5 provides the concluding remarks and future research directions.
4. Numerical Simulation
In the simulation, the expected profit of a retailer for a month,
Nt = 744 h, is optimized by the multi-stage stochastic model proposed in this study. In addition, uncertainty parameters are updated every day on an hourly basis, and therefore,
Nd = 24 h. Regarding the customer’s responsiveness,
is assumed to be −0.03; other simulation conditions are provided in
Table 2.
Uncertainties of demand and PV generation are considered based on the forecast errors and each probability distribution function of forecast error is modeled as an independent normal distribution [
41,
42,
43,
44,
45,
46,
47]. In each normal distribution, means are set equal to the forecasted values and the standard deviations are a specific percentage of the mean value. For the forecasted values of demand and PV generation, day-ahead predicted demand data scaled as 0.5% of the total demand of PJM in July 2017 [
48] and forecasted PV generation data scaled as 5% of the total installed capacity of Belgium in July 2017 [
49] are used, respectively. The standard deviations are 3% and 10% of the means, respectively [
41]. In addition, day-ahead locational marginal prices obtained from PJM in July 2017 are set as the forecasted values of both energy and band prices [
48]. Uncertainty of price is derived by forecast errors of demand and PV generation weighted by Pearson correlation coefficient, respectively, and also the randomness of price itself is added as expressed in Equation (39).
Based on the probability distribution, 1000 scenarios are generated using the Monte Carlo method with even probability. To reduce computational burden, 1000 original scenarios are reduced to 10 through the backward scenario reduction algorithm, the Kantorovich distance method [
50]. One of ten representative scenarios associated with each uncertain parameter is depicted in
Figure 5 and each scenario probability is listed in
Table 3.
Furthermore, we assume that some demand peaks that were initially unpredicted emerge over time. These demand peak values are presented in
Table 4. Moreover, as explained in
Section 3.3, electricity prices are updated daily based on the price announced in the day-ahead PXFC market.
To maximize profit, a retailer optimizes bidding amount of energy and band capacity in the PXFC market and when issuing critical events in the retail market. The resulting optimal bidding amount of hourly energy and band capacity are represented by the solid line and dotted line, respectively, in
Figure 6. In addition, the positive and negative violations that exceed the band capacity are depicted in
Figure 7. A penalty price is imposed on the amount of violation.
4.1. Comparison Between the Mean-Value Deterministic Model and Multi-Stage Stochastic Model
This section analyzes the optimization results of the mean-value deterministic model and multi-stage stochastic model to validate the effectiveness of the proposed model. First, profits of the retailer calculated for one month in these two models are compared in
Table 5. Total profit is greater by 2.45%, revenue is higher, and all costs are lower in the multi-stage model than the mean-value deterministic model.
Table 6 shows the optimal critical event schedule of each model. The intuitive explanation for the notification of critical events at time periods 255, 256, and 257 in the multi-stage model is that the optimal schedule reflects the re-predicted demand peaks. Analytically, a shift in critical event time can be explained by the profit index (PI), defined in Park et al. [
18]. The PI is an additional profit for the retailer when a critical event is triggered for each time period
t.
Table 7 lists the PI results for different critical event schedules in each model. The PIs are higher for time periods 255, 256, and 475 than for time periods are 281, 282, and 472 in both models. In other words, periodically updating market information with the most recent data can trigger critical events more accurately, and maximize profit, with the multi-stage stochastic model compared to the mean-value deterministic model.
4.2. Impact of Input Parameters Change
This section investigates the strategy and profit of a retailer with the variation of four input parameters. First, as shown in
Figure 8a, the profit of the retailer monotonically decreases with an increase in the absolute value of price elasticity of demand (
). An increase in
means that demand further reduces in response to a variation in price, as expressed in Equation (4).
Table 8 summarizes the total demand reduction for different price elasticities of the demand. The decrease in profit can be explained in relation to the demand reduction as follows. It is because sales revenue, which applies a high peak rate to the remaining demand when a critical event is issued, is greater than the overall cost savings in the PXFC market due to demand reduction. Consequently, the profit of the retailer decreases as demand is further reduced.
Second, as shown in
Figure 8b, the profit of the retailer increases as the maximum critical event duration (
DCPP) increases. This is because the longer a critical event lasts after its triggering, the more relaxed the critical event constraint is. In
Figure 8b, the retailer’s profit appears to be the same when
DCPP is 3 h and 4 h. However, precisely when
DCPP is 4 h to 5 h, the profit increases slightly and the optimal critical event schedule changes, as shown in
Table 9. The optimal critical event schedule for different values of
DCPP is also summarized in
Table 9, with successive critical events separated by a slash. Eventually, the profit converges when
DCPP exceeds 5 h because there is no need to maintain critical event.
Third, we analyze the retailer’s profit by introducing a PV proportion factor (PF) to adjust the capacity of the self-generation PV facility. In the case of a reference PV capacity, PF is set as 1.0. As shown in
Figure 8c, an increase in the PF leads to a monotonic increase in the retailer’s profit. This is because the amount of energy that should be purchased in the PXFC market decreases as PF increases, as seen in
Figure 9. However, with the increasing fluctuation in PV generation as the PF rises, the retailer purchases more band capacity to prevent an extreme increase in penalty costs. In other words, when the probability of exceeding the band capacity increases, the retailer will establish a more conservative bidding strategy.
Figure 10 illustrates optimal purchases of energy and band capacities for different PFs.
Finally, a higher penalty price leads to a monotonic decrease in the retailer’s profit, as shown in
Figure 8d. Similar to the case of PF, if the penalty price increases, the retailer purchases band capacity more conservatively to prevent a sharp increase in penalty costs. From
Table 10, we can see that if the penalty price increases by 2 times, from 125 to 250
$/MWh, retailer purchases 1.62 times more band capacity and the penalty cost increases by 1.21 times.
4.3. Sensitivity Analysis
In this section, we conduct a sensitivity analysis to assess the impact of each uncertain parameter on the retailer’s profit. First, we define an uncertainty factor (UF) that scales the standard deviation of each uncertain parameter so that the degree of uncertainty can be changed. UF of price, demand and PV generation are represented as
,
, and
respectively. By Equation (39),
, which refers to
at time period
t, can be derived as a function of
and
as in Equation (40). To make one representative value over the time period, the average value of
can be obtained from Equation (41). In
Table 11,
is calculated for each pair of
and
and it can be observed that the
has a greater impact on
than
. Profit by changing UF for uncertain parameters of demand and PV generation is shown in
Figure 11 and
Table 12. Since the PV generation is extremely small compared to demand, as seen in
Figure 11, the increase in the
does not have a significant impact on the retailer’s profit. There are even intervals where the retailer’s profit increases as the
increases. This is because the retailer revises the optimal schedule by reflecting the increased uncertainties. Thus, the retailer maintains a consistent profit level. In contrast, the increased
results in a significant decrease in the retailer’s profit, as shown in
Table 12. In other words, in this case, a shift in the optimal critical event schedule does not prevent profit reduction.
4.4. Effect of the Energy Storage System
In this section, the effect of the energy storage system (ESS) on the proposed optimization problem is investigated. For the ESS operation, a rule-based strategy is applied, in which ESS is charged when the energy price is low, and discharged when the energy price is high, to reduce the purchasing cost of energy in the PXFC market. Parameters of ESS are summarized in
Table 13. It is assumed that the number of cycles per day is limited to one and the initial stored energy is 1 MWh.
Figure 12 illustrates the power of ESS based on the rule-based strategy for 24 h. From the operation result, the retailer’s profit increases by 3.22% with the same optimal critical event schedule compared to the case when ESS is not utilized as shown in
Table 14.
5. Conclusions
This paper proposes an optimal CPP operation for a retailer with a self-generation PV facility. The problem is formulated as an MINLP model to optimize both the bidding strategy in the PXFC market and CPP operation in the retail market. In addition, an MSSP model is established for the retailer to address the uncertainties of electricity price, demand, and PV generation. By using the MSSP model, solutions can be determined sequentially with realization of uncertainties. The RH method is used to simulate the problem to change the decisions iteratively. Furthermore, to ensure global optimality, the MINLP is transformed into an MILP through three steps. In a numerical simulation, the proposed multi-stage stochastic model yields higher total profit than the mean-value deterministic model. In addition, we investigate the optimal strategy of the retailer by changing four input parameters: Price elasticity of demand, maximum critical event duration, PV PF, and penalty price. Sensitivity analysis confirms that a retailer’s profit is more sensitive to the uncertainty of demand than that of PV generation. Finally, the effect of the energy storage system on the proposed optimization problem is investigated.
However, some challenges still remain for future research, such as accurately modeling the characteristics of residential demands. For example, we can consider the payback phenomenon. If demand is reduced, some of it will be shifted to another time period. It is difficult to stochastically model the extent to which or the time when reduced amount of demand will shift. However, as mentioned above, demand uncertainty has the greatest impact on a retailer’s profit. Therefore, future studies can consider the payback phenomenon stochastically for a more practical implementation of CPP. Also, as many previous studies have demonstrated that curtailment of energy generation from RESs can obtain an economic benefit, an effect of active control of RESs can be highlighted in the proposed problem as future work. Finally, the same values are employed for energy and band capacity prices in this study. It is because the price difference between the two products may lead to complex strategic behavior of the participants in the PXFC market. Therefore, the proposed method can be extended to cope with the price difference between energy and band capacity in the future study.