1. Introduction
The electric power system is experiencing tremendous transformation globally to meet the rising electricity demand and combat greenhouse emissions. Due to its low operational costs and emission-free nature, adopting renewable energy has been crucial for achieving carbon neutrality in electric power grids. Variable renewable energy sources (VREs), mainly wind and solar, currently comprise most of the world’s electricity sources due to their abundant resource potential and local availability [
1,
2]. However, due to their intrinsic variability, the widespread incorporation of VREs has imposed massive complexity in the power systems’ capacity sizing and operational planning [
3]. With the limited capacity of the ESS to smoothen out the surplus or deficient power in the energy system, it is complicated to balance power demand and supply at high-time resolutions. Due to VREs’ limited dispatchability, the need to improve the power system planning approaches and incorporate more flexible resources and technologies to enhance system elasticity and flexibility is becoming increasingly important [
4]. Flexibility is the power system’s capacity to balance electricity supply and demand when highly variable energy resources are incorporated [
5]. Notably, ESS provides the necessary temporal support to offset the variabilities of VRE, and dispatchable power plants a quick ramping up generation supplement in times of low wind and solar [
6]. The transmission networks offer geographical smoothing to balance the supply and demand in the system [
7,
8]. Demand-side management strategies regulate the FDRs to address the mismatch between the load demand and power output of the VREs [
9]. Thus, including demand response strategies in a joint long-term component sizing and operation planning model is essential to realize a high VREs-based power system.
The recent innovative grid concept has injected more intelligence into grid management practices, especially in the face of changing load patterns and the proliferation of energy supply from renewable energy injection [
10]. Thus, existing grid infrastructures are well equipped with information and communication technologies for real-time monitoring and prompt control of power system equipment. As a result, the incorporation of smartness into grid infrastructures is now making the concept of demand-side management (DSM) a vital component of the electricity infrastructures of many nations all over the world [
11]. While DSM is a broad concept, demand response programs (DRP) are underlying energy market strategies whereby consumers’ electricity consumption behaviors are tactically altered regarding the time of demand for using their flexible loads [
12]. The specific goal of demand-side management is to ensure that the most suitable and economically viable operating conditions are achieved based on the information on the available supply (generating) capacity, the demand (load) requirements, and the energy market dynamics per time. Thus, specific changes in customers’ electricity usage patterns are achieved by utility companies providing some incentivization through friendly tariff packages such as flexible payments and lucrative prices for altered load demand patterns and the consequently consumed electricity units. With an appropriately designed DRP scheme, the effective grid planning and operation cost resulting from additional generation and transmission capacity expansion towards meeting increased load demand can be grossly reduced [
13]. Fundamentally, existing DRP models can be classified as either rate/price-based or incentive/event-based demand response models under which there are different DRP types, namely day-ahead pricing (DHP), critical peak pricing (CPP), time of use (TOU), real-time pricing (RTP), etc. [
14,
15].
Demand response programs (DRPs) are a crucial enabling framework in maintaining the power grid’s reliability and stability through electricity market participation. DRP operation strategies provide a mechanism for controlling or rescheduling flexible power demands to increase or decrease capacity during specific periods, for example, to take advantage of high VRE generation or lessen peak demand in the system [
16]. As mentioned above, these techniques make it possible to incorporate large amounts of VREs into the power network in the most economical way possible [
17]. Depending on the motivation strategies of the DRP adopted, the subscribed customers (both residential [
18] and industrial [
19]) are enticed to regulate the power consumption of their flexible demand resource when the total power generation is insufficient and vice versa. Through rebates or incentive payments, consumers are rewarded for reducing their peak demands or shifting part of their daily energy consumption from peak to off-peak hours. As substantiated in numerous literature, the most efficient approach to implementing an effective DRP is that it influences most consumers to participate in real-time power-dispatching transactions, ensuring that the system’s overall efficiency and stability are improved [
20]. The DRPs have numerous potential benefits in all aspects of power system planning, i.e., operation planning [
21] and long-term capacity and expansion planning [
22]. By implementing a well-devised DRP to utilize the installed capacity optimally, it is possible to postpone investing in additional new generating units, delay non-essential network upgrades, etc. Hence, it is essential to incorporate DRP strategies into the operation and capacity planning models. Moreover, implementing suitable demand response (DR) programs and load shifting techniques can enhance the technical performances of energy system networks towards resolving potential overvoltage issues in the network [
23].
Many economic and technical power system planning studies in the literature consider multi-stage or segregated planning models: long-term investment models as well as operation or unit commitment models to determine the optimal operation planning and demand side management to determine the best electricity market operation strategies with varied simulation time frames [
24]. At the long-term investment level, the planning model specifically handles the capacity planning of the generation and transmission components and provides requisite insights into every possible future scenario and the techno-economic consequence of various technologies on the power system, which often serves as a benchmarking tool for policymakers to make optimal investment decisions [
25]. However, these models often consider a considerably extended time frame, i.e., 5–25 years, disregarding short-term constraints, which is crucial in power system operation planning with a high penetration of VREs. The outcome of such models may often give oversimplified conclusions that adversely affect the planning decisions that often undervalue the potential benefits of flexible resources while overestimating the system’s VRE capacity as their power output is time-dependent. On the other hand, studies on short-term system planning consider generation scheduling (economic dispatch) and electricity market strategies on a high temporal resolution (for a few seconds to days), making it feasible to capture the dynamics and manage the variability of the VREs and load demand for increasing the renewable energy penetration level. Moreover, optimizing capacity planning for renewable energy systems with demand response programs can help achieve cost savings by strategically shifting electricity consumption to periods of lower demand or higher renewable energy generation. Consequently, effective capacity planning ensures the adequate generation and reliability of supply at a reasonable service cost.
The authors in [
26] discussed the impact of the co-planning of DR with component sizing on the feasibility of hybrid renewable energy systems for an isolated remote community in Northern Canada that primarily relies on fossil fuels. This study emphasizes the importance of integrating demand response strategies to maximize the techno-economic benefits of the system using a fuzzy inference system. In [
27], a new structure called an off-grid integrated energy system (OIES) for meeting the energy demands of specific customers was proposed using a mixed-integer quadratic constrained programming (MIQCP) model. A key observation in this work is that different segregated models were deployed to achieve the specific aspects of work before combining their outputs for different configurations; this study does not consider the effects of demand response. In [
28], a planning model that considers the influence of price-based and incentive-based demand response strategies on capacity allocation in a grid-connected integrated energy system is proposed using the least cost minimization approach with mixed-integer linear programming. This study highlighted the benefits of demand response regarding economic savings and the effect on the environment considering emissions. The authors in [
29] presented a methodology for planning and operating a hybrid renewable energy system (HRES) for effective energy management. The energy management strategies are developed to minimize operating and environmental costs while meeting the electricity demand and adequately considering future generation and demand response. Research studies have revealed that incorporating short-term operation limits in the long-term investment planning model significantly mitigates the shortcomings of the above-segregated models using soft-linking techniques [
30,
31].
In [
32], a systematic review of integrated resource operation, planning, and design for off-grid energy systems was presented. The authors highlighted the need for and the complexity involved in simultaneous investment planning and operational scheduling for techno-economic efficiency. The integrated implementation of demand response (DR) in microgrid planning using the concept of bottom-up microgrid planning for encouraging energy end-user participation is presented in [
33]. In earlier work, we developed a comprehensive planning and operation framework for an efficient energy supply system for isolated microgrid applications; this study presented the concept of the whole integration of capacity planning and operation scheduling with different demand response strategies [
34]. In that framework, a single model that combined integrated capacity sizing and operational planning, considering various demand-side management strategies, was achieved based on the least cost approach using MATLAB’s mixed-integer linear programming algorithm. However, economic load re-modeling using demand-side management strategies based on power imbalances in the system and economic scheduling of the available generation outputs can be considered to achieve a more optimally techno-economic efficient energy supply system. Hence, this study proposes an integrated model that considers the strategic introduction of the short-term operational details into an optimal component planning model for the significant uptake of renewable energy resources. Based on the available generation capacity of the VREs, the impacts of appropriately devised demand response programs (power capacity-based dynamic pricing—PCDP) on the reliability level of the microgrid and the corresponding effects on the energy storage dynamics are estimated. The design and operation procedure of the proposed joint model addresses the economic, technical, and time-related dynamics of the system components and VREs’ variability. This collaborative planning and scheduling approach provides additional flexibility measures for managing the variability of the VREs and the load demand towards achieving the most techno-economically feasible grid-independent renewable energy-based microgrid. Thus, the specific contributions of this work are itemized as follows:
A joint bi-objective optimization planning approach that incorporates capacity sizing and operational planning into a single objective model with the inclusion of demand response strategies in realizing a feasible VREs-based power system is proposed and investigated.
The cost-benefit advantage of considering CPP DRP, EDRP, and PCDP DRP in determining the most effective techno-economic planning pathways for achieving a zero-carbon grid-independent VREs-based microgrid is compared.
An energy storage dependency (ESD) assessment metric that quantitatively evaluates and indicates the reliance of a VREs-based microgrid system on energy storage systems to satisfy the load demand at a pre-determined reliability index is proposed and investigated.
An enhanced time-based PCDP DRP program is devised that schedules the flexible demand resources to offset the surplus and deficit power imbalances in a VREs-based microgrid system is formulated and investigated. The PCDP DRP technique provides the necessary flexibility from the demand side to mitigate the variability of VREs’ output power and the load demand, thereby guaranteeing a cost-effective optimal planning approach for a grid-independent VREs-based microgrid is realized.
The remaining parts of this paper are organized as follows:
Section 2 presents the system modeling and methodology;
Section 3 provides a detailed formulation of the flexible demand resources and modeling of demand response programs. Problem formulations and optimization models are discussed in
Section 4. The simulation parameter specifications and the case study description are given in
Section 5, while
Section 6 discusses the simulation results. Finally,
Section 7 concludes the work.
4. Problem Formulation and Epsilon-Constraint Mixed-Integer Linear Programming
The solution of the formulated optimization problem is achieved using an epsilon-constraint mixed-integer linear programming (
) model, which determines the optimal solution for the system. The epsilon-constraint methodology selects one objective (
) as the main objective, and the other objectives are converted into constraints (
) [
38]. The boundary setpoints for the
constraints are then systematically varied to obtain optimal solutions [
39]. The
-MILP is a widely used technique for solving bi-objective optimization as a single-objective constrained problem. The superior characteristics of the
-constraint MILP optimization were shown and proved to be capable of producing a credible and verifiable solution, as substantiated in the literature for various case studies involving VREs-based systems optimization, such as smart buildings [
40], integrated energy systems [
40,
41], remote islands [
42], off-grid power systems, etc. Thus, this study adopted the bi-objective optimization model for the microgrid’s capacity sizing and short-term operating planning with or without considering the DRP while maximizing the system’s reliability using the loss of power probability (LPSP). The optimization problem is solved in the MATLAB environment.
The
can be expressed as:
where
i and
j are integers and continuous decision variables, respectively, representing the optimal solution for the proposed system. The constraints are modeled as linear and expressed as inequality
A and equality
matrices with their related
and
b column vectors, respectively;
and
are the decision variables’ upper and lower decision variables’ boundary limits. In this work, the optimization is accomplished using the MATLAB
® INTLINPROG toolbox.
4.1. Objective Function
The main objective is to minimize the total annualized costs (
), which entails the initial acquisition (
), maintenance, replacement (
), and operating costs in both the short-term and long-term component sizing planning time frame considering the various prospects of demand response programs. Thus, the overall objective function of the proposed microgrid configuration is as expressed below:
Subscript
z indexes the
system component and
denotes the optimum size of each system’s component which is part of the decision variables to be determined by the optimization program.
d is the discount rate, and
Y is the lifetime of the project.
is the capital recovery factor, which is a function of the project lifetime
Y and discount rate, as elaborated below:
The discount rate
d is computed from the annual interest rate
i and inflation rate
f using Equation (
18).
The first term of (
16) is the net present cost which comprises the capital costs and the discounted replacement, and the yearly operation and maintenance costs considered over the entire project lifetime of each system component. The capital recovery factor converts the net present costs to the equivalent annualized costs, as illustrated in (
17).
The second part of the objective function is the total annual operational cost associated with the electricity market design scheme. Depending on the operational strategy of the demand response option selected, the associated costs for the curtailment or short-term rescheduling of the elastic load demand are given by:
where
is the curtailed load demand based on the nominated DRP strategy, and
denotes the incentive payments or rebates received by the consumer as settlements for adherence to load curtailment or the reduction requirements set by the utility or the system operator.
4.2. Constraints
Power balance constraints:
The total generated power at any given time (h) should always meet the expected system load demand with or without DRP in all scenarios considered.
Battery constraints:
The minimum and maximum boundary limits bound the state of charge of the battery system.
Equation (
22) denotes the maximum discharging (
) and charging power (
) limits of the battery system:
Flexible demand resource constraints:
flexible demand resource (
) rescheduling or reductions must not exceed the maximum (
) and minimum (
) allowable capacities specified by the DRP scheme chosen at any given time (
h).
Set the electricity price constraint:
The new set price () determined by the DRP pricing scheme must be within the maximum () and minimum () allowable electricity price limits specified by DRP at any given time (h).
constraint:
where LPSP is loss of power supply probability.
6. Results and Discussions
The following are the simulation results and analyses of the four cases under investigation in this study;
6.1. Case 1: Component Sizing and Operation Planning without Considering Demand Response
In this case, the demand response program was not considered when simulating and evaluating the microgrid component sizing and operation planning for the system under study. As a result, the load demand must be met by the power generation output of the PV and WT, and whatever deficit or surplus power remains in the system will be regulated by the BESS. This scenario is considered the system’s base case. The optimum component size of the simulation under various reliabilities (LPSP = 0%, 2.5%, and 5%) and all related costs are summarized in
Table 3.
Figure 3 shows the power dispatch of the various system units’ contribution to meet the load demand at the LPSP = 0%.
Figure 4 compares the BESS utilization based on SOC at different levels of reliability (LPSP = 0%, 2.5%, and 5%).
The output power of the optimum system component sizes selected by the program can meet the load demand per the expected system requirements (LPSP = 0%), as shown in
Figure 3. Since DRP is not considered, there are no costs related to the electricity market operation. As can be seen, there is a direct correlation between ESD, total yearly costs, reliability index, and system component size. Higher system reliability requirements necessitate larger PV, WT, and BESS system component sizes, increasing the total system cost (TAC). As per the results above, the BESS is the most expensive system component, which is about 45% of the total annualized costs of the microgrid due to the system’s higher dependency on ESS, as indicated by the high values of the ESD.
6.2. Case 2: VREs-Based Microgrid System’s Component Sizing and Operation Planning Considering CPP DRP
In this case, the potential benefit of CPP DRP on the capacity sizing and operation optimization for microgrids is explored. The system components’ sizes and costs are interrelated similar to case 1. The optimum component size of the simulation under various reliabilities (LPSP = 0%, 2.5%, and 5%) and all related costs are summarized in
Table 4.
Figure 5 shows the power dispatch of the various system units’ contributions to meet the load demand considering CPP DRP at the LPSP = 0%.
Figure 6 compares the BESS utilization based on the state of charge at different levels of reliability (LPSP = 0%, 2.5%, and 5%).
The simulation results show that the higher the reliability requirements are for the system, the more expensive the system is, as depicted by the TAC values; this is because the reliability index (LPSP) and total system costs (TACs) have conflicting goals. Similarly, the more the system is dependent on ESS, the more expensive the system is; this is depicted by the higher percentage values of the ESD, which translates into higher costs for the BESS, as shown in
Table 4.
6.3. VREs-Based Microgrid System’s Component Sizing and Operation Planning Considering EDRP DRP
In this case, capacity sizing and operation planning are simulated and evaluated while considering EDRP at various reliability requirements.
Table 5 summarizes the system components’ sizes and related costs for the optimum microgrid configurations under varying reliability requirements (LPSP = 0%, 2.5%, and 5%). According to
Table 5, unlike scenario 1 and 2, the total annualized costs (TACs) are composed of both expenses related to the equipment sizing and the operation strategy of the demand response program considered. In this case, the DRP operation costs are mainly due to incentive payments to customers participating in the EDRP program.
Figure 7 shows the power output dispatch profile for the WT and PV and the BESS’s charging and discharging power.
Figure 8 compares the BESS utilization at different levels of reliability (LPSP = 0%, 2.5%, and 5%) based on the state of charge.
6.4. VREs-Based Microgrid System’s Component Sizing and Operation Planning Considering PCDP DRP
This simulation scenario examines the potential benefits of the proposed PCDP DRP on capacity sizing and operation planning.
Table 6 summarizes the system components’ sizes and related costs for the optimum microgrid configurations under varying reliability requirements (LPSP = 0%, 2.5%, and 5%). The generation power output profile of the WT, PV, and BESS contribution in meeting the microgrid load demand is shown in
Figure 9.
Figure 10 compares the BESS utilization at different levels of reliability (LPSP = 0%, 2.5%, and 5%) based on the state of charge.
From the results, a relaxation of the system reliability requirement from 0% to 2.5% and 5% translates to a, respectively, 9% and 13% decrease in the total annualized costs (TACs); this is due to load reduction by the consumers during times of peak load demand and the power output of the generating units cannot meet the required capacity.
Furthermore, it is vital to note that the PCDP electricity pricing strategy is devised such that the flexible demand resources are shifted in such a way as to minimize both the shortage and surplus power in the system, thereby increasing the overall system efficiency. Consequently, the PCDP DRP techniques guarantee that the mismatch between the total output power of the VREs and the load demand is minimized; thus, the ESD is minimum in all cases, resulting in a significant reduction in the BESS capacity requirement.
Figure 11 shows the new electricity price versus the power mismatch in the system due to the impact of PCDP DRP program implementation.
6.5. Techno-Economic Comparison for Each of the Scenarios Based on Demand Response Program Options at Maximum System Reliability (LPSP = 0%)
The techno-economic benefits of each DRP in the long-term investment and short-term microgrid planning for the proposed VREs-based system are assessed and analyzed in this section for the four cases considered. The techno-economic comparison for each of the cases based on DRP options at maximum system reliability (LPSP = 0%) is summarized in
Table 7.
Case 1 is the most expensive system compared to all four cases, with a total annualized cost of USD /year. Since there is no inclusion of demand-side management in this case and the VREs’ power output cannot be dispatched or controlled, the battery energy is the only flexible resource depended upon to address the mismatch between the generation and the load demand. Thus, a BESS capacity of approximately 6400 kWh is needed to serve the load demand as per the set reliability requirement, consequently, a high ESD value of about 13%. It is worth noting that the BESS is the most expensive component amounting to about 45% of the TAC; thus, the need for DRP to lessen the reliance of the BESS in the system. Considering the DRP in case 2, the benefit of the CPP DRP results in a cost saving of approximately 5% of the total annualized costs compared to the reference case, from US (without DRP) to (with CPP DRP considered).
The cost reduction, in this case, is attributed to a drop in ESD requirement from 13% to 12%. Since the CPP DRP reduces the load demand from the maximum peak load periods to off-peak periods, thus, a lower BESS size is required. Consequently, a BESS capacity of 5700 kWh is sufficient to meet the load demand per set system reliability, which is about a 10% decrease in BESS size compared to the reference case. However, implementing the CPP DRP results in a load curtailment of about 0.027% of the total load demand by the consumers. This is because the CPP DRP penalizes the electricity consumers by enforcing high pricing during peak periods; the electricity consumers are therefore compelled to curtail or minimize their power consumption due to cutting down electricity costs, which is an undesirable effect.
Compared to the reference case (case 1), the implementation of EDRP in case 3 has various cost–benefits to long-term investment planning, as shown in
Table 7. The resulting load profile, due to the EDRP DRP adoption, yields a 2% decrease in ESD from 13% to 11% as the FDRs are being shifted from the high peak period with high ESS dependencies to the low peak period where there are surplus generations from the VREs; this results in a 3% decrease in the TAC compared to case 1. It is worth noting that compared to the previous case of CPP DRP, the TAC is slightly higher due to the cost of running the EDRP electricity market strategy, as the system operators have to pay consumers some incentives for complying with the requirement to reduce energy consumption during EDRP peak load periods. Furthermore, it could be observed that EDRP also results in a 0.024% reduction in the overall load demand, which is also an undesired consequence.
The techno-economic aspect of PCDR DRP is noted to significantly impact both long-term investment and operational planning. From the electricity consumer side, the DRP participating clients have a cost savings of about 2.54% for an equal amount of consumed energy compared to the reference case (case 1); since PCDP DRP gives the most considerable cost saving compared to the two other cases (case 3 with EDRP and case 2 with CPP) and the aggregate energy consumption equals the total demand of the base case, meaning that there is no load demand curtailment; thus, this is the most preferred demand response program. As a result of implementing the DPCP pricing scheme, consumers have sufficient motivation to shift their load demand profile from high electricity price periods to low price periods, thereby fetching the lowest prices; thus, the overall cost saving compared to the three other cases.
From a generation planning perspective, deploying the PCDP DRP program yields a new load profile optimal for the day-to-day operation and long-term investment planning. As the PCDP pricing scheme encourages consumers to time shift the FDRs from peak load periods to coincide with peak periods of VREs generation, the system dependency on the ESS is drastically reduced, thus resulting in a lower value of ESD of about 9%. Therefore, the capacity of the BESS needed to balance the power mismatch in the system is significantly reduced by 23%, and thus, the TAC is also reduced by 10%.
Figure 12 demonstrates and compares the impacts of DRPs in decreasing the mismatch between the surplus and shortage of power in the system due to the variability of VREs generated power and load demand.