Battery-supercapacitor hybrid energy storage system in standalone DC microgrids: areview
Abstract
Global energy challenges have driven the adoption of renewable energy sources. Usually, an intelligent energy and battery management system is deployed to harness the renewable energy sources efficiently, whilst maintaining the reliability and robustness of the power system. In recent years, the battery-supercapacitor based hybrid energy storage system (HESS) has been proposed to mitigate the impact of dynamic power exchanges on battery's lifespan. This study reviews and discusses the technological advancements and developments of battery-supercapacitor based HESS in standalone micro-grid system. The system topology and the energy management and control strategies are compared. The study also discusses the technical complexity and economic sustainability of a standalone micro-grid system. A case study of a standalone photovoltaic-based micro-grid with HESS is presented.
1 Introduction
Global energy challenges and their impact on the environment have accelerated the adoption of renewable energy sources and development of smart and efficient micro-grid technologies [1, 2]. Low voltage micro-grid in particular has attracted increasing attentions from researchers. Micro-grid is a small-scaled autonomous power grid system that consists of multiple energy generations from renewable and non-renewables resources, energy storage systems (ESS) and power electronic converters. Micro-grid can be operated either in standalone mode or connected to the utility grid [3-6].
A key advantage of micro-grid is that it allows power generation and supply to remote isolated community without the need for costly and inefficient long-distance high-voltage transmission and distribution infrastructures [7, 8]. However, maintaining a robust, high quality and reliable standalone micro-grid is challenging due to its relatively small capacity and the intermittent nature of the renewable energy resources [9]. Various developments have been carried out to improve the power quality and reliability of the micro-grids, including the introduction of novel micro-grid topologies [10-12] and state-of-the-art power management and control strategies [13-16].
Unlike the grid-connected micro-grids that have virtually unlimited support from the high inertia power generators, standalone micro-grids leverage heavily on its ESS to balance the mismatch between the power it generates and the power being consumed [17]. The ESS acts as buffer to store surplus energy and supply it back to the system when needed. ESS in standalone micro-grid also play an important role in regulating instantaneous power variations and maintaining power quality [18]. Table 1 summarises the different ESS elements and their key characteristics.
| Energy storage system | Energy density | Power density | Cycle life | Response time | Cost |
|---|---|---|---|---|---|
| chemical battery | high | low | short | medium | low |
| sodium-sulphur (NaS) battery | medium | low | short | slow | medium |
| flywheel | low | high | long | fast | high |
| supercapacitor | low | high | long | fast | medium |
| superconducting magnetic energy storage | medium | high | long | fast | high |
In standalone micro-grid, the power flows in and out of the ESS elements varies widely depending on the instantaneous power generation and load condition [20]. In general, the power exchanges in ESS can be categorised into high-frequency components such as sudden surge in power demand or intermittent solar power generation on a cloudy day, and the low-frequency components such as natural behaviour of renewable energy resources or daily average energy consumption pattern [21]. High-frequency power exchanges generally require ESS elements with fast response time, while low-frequency power exchanges require high energy density ESS elements.
Based on the characteristics of the different ESS elements shown in Table 1, none of them has the characteristics to respond optimally to both high and low frequency power exchanges [22]. One way to get around this limitation is by combining multiple types of energy storage elements to form a hybrid ESS (HESS). A battery-supercapacitor combination has been considered in most HESS developments because of their availability, similarity in working principle, relatively low cost and most importantly, they complement each other limitations very effectively.
The automotive industry has developed HESS for electrically driven vehicles. HESS had shown great improvement in maximising the energy recovered from regenerative braking, increasing the rate of charging and prolonging the service life of battery by reducing the strain of deep discharge [23]. The development of HESS for residential energy storage applications is beginning to generate positive outcomes as well [24-26]. HESS is typically connected to the power network via AC or DC coupling. Power converters are used to control the power flow among different ESS elements [27-29]. Depending on the complexity of the control strategies, the use of power converters and microcontrollers can be costly [30]. Hence, the trade-off between economic feasibility and technical advantages exist and it is crucial in determining the financial and technical sustainability of micro-grid implementation.
Various battery-supercapacitor HESS topologies have been proposed [31, 32]. Besides the topology, the energy management and control strategies used in HESS are crucial in maximising efficiency, energy throughput and lifespan of the energy storage elements [33-37]. This paper reviews the current trends of battery-supercapacitor HESS used in standalone micro-grid.
Section 2 presents the developments of battery-supercapacitor HESS topology for high-energy storage applications with a comprehensive analysis of different HESS in standalone micro-grid. Section 3 reviews the existing energy management strategies including control goals, power allocation strategies and safety measures. In Section 4, a case study of a standalone photovoltaic-based micro-grid with different HESS topologies is presented and compared. Section 5 gives a comprehensive review of the different control algorithms used in energy management system (EMS) and an evaluation of their effectiveness as well as economic and technical viability. Future trend of HESS development in standalone micro-grid is also presented. Finally, the paper is concluded in Section 6.
2 Battery-supercapacitor HESS topologies
In battery-supercapacitor HESS, the two ESS elements can be coupled to either a common DC or AC bus [38-40]. For standalone micro-grid, common DC bus is the preferred choice due to various reasons [41, 42]. First, most ESS elements and renewable energy generators operate in DC voltage. Therefore, maintaining a DC bus minimises the needs of power converter [43]. Second, DC bus does not require synchronisation which greatly reduces the complexity of the overall system [44, 45]. As a result, DC coupling is more efficient and lower cost than equivalent AC bus systems [46-48].
In general, battery-supercapacitor HESS can be categorised based on their connection topology as depicted in Fig. 1 [49, 50].
Classification of the battery-supercapacitor HESS topologies
2.1 Passive HESS
Passive connection of battery and supercapacitor to the DC bus is the simplest and cheapest HESS topology. It has been shown to effectively suppress transient current under pulse load conditions, increase the peak power and reduce internal losses [51-54]. As shown in Fig. 2, the battery and supercapacitor are connected to the DC bus directly. They share the same terminal voltage that depends on the state-of-charge (SoC) and charge/discharge characteristic of battery. In some rural micro-grid applications, the battery capacity is sized up to five days as reserve without any external source of energy [55]. Consequently, most of the time the battery will be cycled with relatively low depth-of-discharge (DoD) and charged/discharged in a relatively low C-rate. As a result, the fluctuation in DC bus voltage will be minimal, ensuring a relatively stable system voltage.
Passive HESS topology
However, the system current will be drawn from or feed into the battery and supercapacitor based on their respective internal resistances [54]. Therefore, the transient power handing capability of the supercapacitor is not optimally utilised. In addition, as the voltage variation of the battery terminal is small, the supercapacitor will not be operating at its full SoC range which results in poor volumetric efficiency [52].
2.2 Semi-active HESS
To make better use of the ESS elements in passive HESS, power electronic converters are included between the ESS elements and DC bus. This allows the power flow to be actively controlled [56]. In semi-active HESS topology, only one of the two ESS elements is actively controlled. Fig. 3a shows a semi-active HESS topology in which only the supercapacitor is interfaced to the DC bus via a bidirectional DC/DC converter, while the battery is directly connected to DC Bus [57]. In this topology, the bidirectional DC/DC converter isolates the supercapacitor from the DC bus and battery terminal. In this setting, the supercapacitor can be operated within a wider range of voltages, which significantly improves the volumetric efficiency. The direct connection of battery also ensures stable DC bus voltage [58]. However, the passive connection of battery unavoidably exposes the battery to fluctuating high current that has negative impact on battery's lifespan [59].
Semi-active HESS topologies
(a) Supercapacitor semi-active HESS topology, (b) Battery semi-active HESS topology
The other semi-active HESS configuration is shown in Fig. 3b. The battery is isolated by a bidirectional DC/DC converter, while the supercapacitor is passively connected to the DC bus [24, 60, 61]. Unlike passive and supercapacitor semi-active HESS topologies, the battery current can be controlled at a relatively gentler manner regardless of the fluctuation in the power demand. The battery terminal voltage is not required to match the DC bus voltage, allowing flexible and efficient sizing and configuration of battery bank [62]. However, the volumetric efficiency of the supercapacitor is low. The linear charge/discharge characteristic of the supercapacitor also causes large fluctuation in the DC bus, which may result in poor power quality and system stability. To maintain a relatively stable DC bus voltage, the capacity of supercapacitor must be extremely large, which leads to high cost.
2.3 Full active HESS
In full active HESS topology, the power flow of battery and supercapacitor are both actively controlled via bidirectional DC/DC converters. This enhances the flexibility of the HESS and improves the overall system performance and cycle life [59]. Two of the most common full active HESS topologies are shown in Fig. 4, namely parallel active HESS and cascaded active HESS.
Active HESS topologies
(a) Parallel active HESS topology, (b) Cascaded active HESS topology
In parallel active HESS topology, both battery and supercapacitor are isolated from the DC bus by bidirectional DC/DC converters as shown in Fig. 4a. Parallel active HESS is one of the most common topologies for grid scaled storage applications which allows full control of both ESS elements [63]. With this topology, the performance, battery life and DC bus stability can be improved through carefully designed control strategy [64]. For instance the battery, as a high energy density ESS, can be programmed to meet the low-frequency power exchanges. The supercapacitor can be programmed to response to the high-frequency power surges and regulate the DC bus voltage. The decoupling of battery and supercapacitor allows both ESS elements to operate at a wider range of SoC that can greatly improve the volumetric efficiency of the HESS.
In cascaded topology, two bidirectional DC/DC converters are cascaded to isolate the battery and supercapacitor from the DC bus, as illustrated in Fig. 4b [65]. The bidirectional DC/DC converter that isolates the battery is normally current controlled to provide smooth power exchange with the battery. This releases the battery from harsh charge/discharge process due to the intermittency of renewable power generation and load. The bidirectional DC/DC converter that isolates the supercapacitor from the DC bus is normally voltage controlled to regulate the DC bus voltage while absorbing the high frequency power exchanges [66]. Since the supercapacitor has wide operating voltage, a large voltage swing between the supercapacitor and DC bus is expected. As a result, the power losses in the DC/DC converter will be higher as it is difficult to maintain efficiency across wide range of operating voltages [65].
As the number of power converter increases, the overall coulombic efficiency of the HESS decreases due to losses in the power converters. The performance of full active HESS system is also extremely reliant on the reliability of the DC/DC converters and their control system.
3 Energy management system
Generally, the objectives of HESS implementation in standalone micro-grid can be grouped into three main categories: (i) optimising micro-grid performance, (ii) enhancing system reliability and (iii) lowering set-up and operating cost. Fig. 5 summarises the objectives. Active HESS topology enables each ESS elements to be optimised through an EMS.
EMS control goals
The role of EMS is to maximise the benefits of HESS. Volumetric and coulombic efficiencies have to be maximised while maintaining system stability and power quality at the DC bus. In terms of system reliability, EMS must ensure robust system operation in all possible loading conditions, protect the ESSs from extreme conditions and extend the useful lifetime of the ESS elements. EMS also needs to ensure that cost of implementation, operation and maintenance are kept low. For instance, scheduling of diesel generator and load can be integrated into the EMS to lower the operating cost.
Fig. 6 depicts a typical EMS structure for HESS in standalone micro-grid applications. In general, the EMS can be divided into two levels: (i) the low-level control system regulates the DC bus voltage and controls the current flowing in and out of ESS elements based on the reference signal generated by high-level control system. (ii) The high-level control system performs power allocation strategy, SoC monitoring and control, and other sophisticated energy management strategies to achieve the set control goals.
Typical EMS structure for standalone PV DC microgrid with parallel active HESS
Zhou et al. [67] adopted the parallel active topology and proposed a modular HESS scheme that splits the single battery bank into multiple smaller battery modules. The supercapacitor module and battery bank modules are interfaced to DC bus using dual-active-bridge bidirectional DC/DC converters. The authors employed a linear filtering approach to remove high frequency power fluctuations and distribute the smooth power demands to each battery modules based on their SoC level. The supercapacitor module will respond the high frequency power exchange through cascaded inner current control loop and outer voltage control loop. A simple SoC management scheme for supercapacitor module is implemented where the battery modules will charge the supercapacitor when the SoC level is lower than a pre-set threshold. The EMS mainly focuses on balancing the charge/discharge current among different battery modules. However, it does not consider the impacts of battery SoC variation in long-term operation, which may affect the system stability and longevity of the battery. Moreover, the proposed modular HESS topology requires a large number of DC/DC converters, leading to significant increase in power loss and set-up cost.
To address the issue of high charge/discharge rate and possible delay in converter's response, Kollimalla et al. [63] adopted the linear filtering approach to decouple the high and low frequency components of the power demand and added a rate limiter to prevent high charge/discharge rate of the battery. An additional compensator is implemented to compensate the slow response of battery. The proposed EMS mainly focuses on regulating the DC bus voltage and mitigating battery stress by limiting the battery current. The technique assumes that all ESS elements work within the acceptable limits throughout the operation. It does not take into consideration the SoC control of the battery. This may cause the battery to experience deep discharge under extreme conditions, which may lead to shorter battery lifespan.
Choi et al. [33] presented an EMS scheme in battery-supercapacitor HESS to achieve two objectives: (i) to minimise the energy loss caused by the internal resistance of the supercapacitor and (ii) to mitigate the fluctuation of current flowing in/out of the battery bank. The author mathematically formulated the two objectives in order to obtain the optimal solution to control the current flow in each ESS elements. The two objectives were formulated into convex optimisation problems, which are norm approximation and penalty function approximation, respectively. The two problems are then combined into a single multi-objectives function. In order to obtain the optimal solution, boundary parameters were determined through multiplicative-increase-additive-decrease principle. The values of the boundary parameters critically determine the feasibility and optimality of the solution. This control strategy only considers the characteristics of ESS elements. It does not consider the interactions between the elements and other components within the micro-grid. Thus, the resulting optimal EMS scheme tends to be for one particular system only.
The above EMS strategies for HESS mainly focus on solving the short-term power demand variations and power sharing between supercapacitor and battery. However, the impact of SoC drift in battery is not addressed. Specifically in micro-grid, seasonal variations in renewable energy generation and load demand must be carefully addressed to ensure reliable operation in all possible loading conditions.
To accurately monitor the battery SoC and to address the long-term SoC variation, Xue et al. [68] proposed an actively controlled, parallel connected battery-supercapacitor HESS in photovoltaic based system that employs a multimode fuzzy-logic power allocator to solve the problem of supply-demand mismatches. Based on the SoC conditions of batteries and supercapacitors, the fuzzy-logic controller selects the appropriate operating mode to allocate power demand to the ESS elements. To avoid overly charging or discharging the ESS elements, the controller allows the power exchange between the battery and supercapacitor and their individual power contribution to be optimally adjusted. The EMS control strategy guarantees all ESSs operate within their safe operating range and compensates transient mismatches between the power generation and demand.
Most EMS rely on a centralised controller to manage the power flow among different ESS elements and DC bus. Therefore, a robust communication link between components is needed. A disruption in the communication link will lead to catastrophic system failure. A hierarchical controller for HESS was proposed in [69] to address this risk. In normal operating mode, the centralised controller allocates power to ESS modules based on their ramp rate. ESS with the highest ramp rate is normally assigned to regulate bus voltage. In the situation where the centralised controller fails, distributed control at individual ESS elements will be activated to ensure continuous micro-grid operation.
Due to the complex and non-linear characteristics of battery and supercapacitor during the charging/discharging operation, simple power allocation method such as linear filtering may not be sufficient to effectively allocate the power demand among the energy storage elements in HESS. Therefore, advance supervisory control algorithms for EMS have been developed. A number of different intelligent and complex control algorithms, such as deterministic rule based strategy, fuzzy logic control, linear programming, genetic algorithms, dynamic programming, neural network, self-adaptive algorithms and etc. have been reported in the literatures [17, 70, 71]. Fig. 7 illustrates an overview of the classification of existing EMS algorithms in HESS. The EMS algorithms are generally categorised into two main classes which are rule-based approaches and optimisation-based approaches [72-74].
Classification of intelligent algorithms for EMS supervisory control in HESS
3.1 Rules-based approach
Rule-based approach controls the power exchange of HESS based on rules that are derived from mathematical models and experiences [75-77]. Rule-based approach is an effective method for real time energy management widely used in HESS applications. In thermostat control strategy, the battery operates with constant power at its optimal efficiency point and it will be turned on or off according to the lower and upper SoC limits. In power follower control strategy, the battery is set as the primary energy storage and the EMS will adjust the battery charge/discharge power that follows the power demand. As a secondary ESS, the supercapacitor covers the difference between the power demand and battery response. Unlike thermostat and power follower control strategy, the state machine control strategy uses multiple rules to control the power flow in HESS. The pre-defined rules can be designed based on the allowable upper and lower SoC limits of supercapacitor and battery, maximum charge and discharge rates, load and generation powers, etc. Based on the real-time operational states of the HESS, power generation and power demand, the algorithm selects the appropriate operation mode to optimise the power distribution between supercapacitor and battery.
The deterministic rule-based concept is widely used due to its simplicity and reliability [75, 78-81]. However, the rules are generally designed based on the initial state of the ESS elements. This may not accurately reflect the actual conditions of the elements in the long run. Therefore, fuzzy logic control strategy shown in Fig. 8 is introduced.
Fuzzy logic control flowchart
The rules are defined based on experiences and empirical evidences. The transition between different rules is determined by the fuzzy-rules and membership functions which results in smoother, more flexible and logical operation compared with deterministic rule-based approaches [68, 82-84]. Fuzzy rule-based control algorithms can be integrated with other intelligent algorithms to form hybrid control strategy which further improve the performance of EMS in HESS [85-88].
3.2 Optimisation-based approach
Optimisation-based EMS employs modern optimisation algorithms, such as linear programming (if the system is convex and could be mathematically represented via a set of linear functions), dynamic programming (both deterministic and stochastic), evolutionary methods such as genetic algorithm, simulated annealing, and particle swarm optimisation [89-93]. These algorithms can be classified into global optimisation (off-line) and real-time optimisation (on-line). Unlike the rule-based approaches, modern optimisation algorithms are much more complex, which require heavy computation capability [72].
4 Case study
Photovoltaic generator is commonly deployed in remote rural sites that are not connected to the utility grid. However, the intermittent nature of solar irradiance and the relatively large fluctuation in load may lead to system instability [22, 34, 94]. Therefore, battery is normally included in photovoltaic system. A case study is presented in this section to demonstrate the effectiveness of HESS in reducing the stress on the battery.
A standalone photovoltaic system with battery-supercapacitor HESS is considered. The system is used to provide electricity to a rural community in Sarawak, Malaysia. A supercapacitor semi-active HESS topology is used, as shown in Fig. 9a. A simple linear filtering power allocation approach is employed in the simulation. The system key parameters are tabulated in Table 2. An actual solar irradiance data recorded on a typical partly cloudy day is used to simulate the photovoltaic power generation. A daily power consumption profile is estimated based on actual survey data from a rural community. The simulated power generation and load profiles are shown in Fig. 9b.
| Parameter | Value |
|---|---|
| PV array power (peak) | 5 kW |
| daily energy consumption | 27.4 kWh |
| battery nominal voltage | 48 V |
| battery capacity | 1000 Ah |
| battery internal resistance | 0.005 Ω |
| supercapacitor capacitance | 1000 F |
| supercapacitor equivalent series resistance | 0.001 Ω |
Case study to demonstrate the effectiveness of HESS in mitigating battery's stress
(a) Standalone PV DC micro-grid with supercapacitor semi-active HESS topology, (b) PV generation and load profiles used in the simulation
To demonstrate the effectiveness of HESS in mitigating battery's stress, a battery-only ESS is also simulated for comparison purpose. The simulation results showing the power exchange in battery for both battery-only and HESS systems are illustrated in Fig. 10. As can be seen from the enlarged view in Fig. 10b, the battery current for system with HESS experienced less severe fluctuation compared with the battery-only system. In addition, the peak current for system with HESS is also reduced compared with system without HESS. As suggested in battery lifetime characteristic studies, these are two of the many factors that accelerate the battery aging and performance deterioration [95, 96]. Thus, minimising these factors can potentially improve the service life of the battery.
Simulated battery current for battery-only and HESS systems
(a) Battery current, and, (b) Enlarged view of battery current
(1)
Normalised battery health cost C(T) for battery-only and HESS systems in 24 h
5 Analysis and discussion
There are varieties of HESS topologies and energy management and control strategies used in micro-grid. Each one of them improves different aspects of the micro-grid. They are selected based on the system requirements, technical and cost constraints and end user expectations. The discussion and analysis in the following subsections focuses on standalone micro-grid for rural communities who are without access to the utility grid.
5.1 HESS topologies and EMS
Most researches on HESS are aimed at reducing the stress on the batteries while maintaining power quality, improving HESS efficiency and lowering set-up cost. For greater controllability, active HESS topology is commonly used. However, it increases system complexity and cost. Passive HESS provides simple, reliable and robust way to mitigate battery stress but at the cost of controllability and performance.
For standalone micro-grid in remote areas where it is the only source of electrical power, system reliability and robustness is prioritised [98]. The semi-active HESS topology, which is relatively simpler than active HESS, is probably the most suited for such application. Stress on the battery bank, which contribute the most to set up and maintenance cost, can be reduced by actively controlling the power flow between energy storage elements.
Apart from the three HESS topologies discussed above, there exist many other sophisticated HESS topologies and control strategies in the literature. In [15], the battery bank is made up of micro-bank modules, each with its own DC/DC converter and EMS. A control strategy that dynamically configures the battery modules is put in place to optimise the use of the modules and for greater system efficiency. Similar concept was proposed in [99, 100], where banks of varied energy storage elements and battery types were used with a global charge allocation algorithm that controls the power flow between the storage banks. With careful usage of power electronic converters, configurable and modular HESS could be one of the future trends in the development of micro-grid power management system. However, it may not be suitable for standalone micro-grid applications in remote area due to the sophisticated and potentially costly system architecture.
5.2 AC coupled and hybrid AC–DC micro-grid
DC coupling is usually used for small-scaled standalone micro-grid in remote rural sites [101-106]. Passive and supercapacitor semi-active HESS are the most commonly used topologies. Whilst they may not be the most efficient topologies, they deliver very robust operation at lower cost [107]. For medium to large-scale micro-grid, AC coupling is commonly used to minimise losses in power transmission [44]. Fig. 12a illustrates a typical AC coupled micro-grid architecture. For greater flexibility, hybrid AC–DC micro-grid that caters for AC and DC power generators and ESSs shown in Fig. 12b is used [38, 92]. The DC/AC and DC/DC converters allows high degree of power flow controllability.
Micro-grid structure with AC bus
(a) AC Coupled micro-grid, (b) Hybrid AC–DC micro-grid
5.3 Limitations and future trends
ESS is a vital element that enables stable and reliable micro-grid operation in the face of fluctuating power generation and load profiles. Therefore, enhancing the performance and reliability of ESS will be a research focus for standalone micro-grids. New battery technologies, such as graphene, lithium-air, aluminium-air and sodium-ion, are anticipated to replace existing batteries with significant improvement in performance and lifespan [108-110]. The hybridisation of ESS and development of EMS are expected to evolve as well to make the most out of battery technologies [111].
The development of HESS is expected to progress in two directions: (i) robust and reliable HESS in small-scale standalone micro-grids specifically for remote or isolated sites, (ii) autonomous and intelligent medium to large-scale grid-connected micro-grids that are part of smartgrid architecture. In general, micro-grids for remote rural electrification will focus on developing HESS that is robust, simple and easy to maintain due to the difficulties in reaching the remote sites.
Conversely, high performance HESS with intelligent EMS and control strategy will be the focus in grid-connected micro-grids or smart-grids. For example, a re-configurable energy storage bank was proposed in [112] to dynamically change the configuration of battery bank to optimise the workload of each cell, leading to improved ESS performance and service life. Other sophisticated ideas of future energy supply and distribution system such as the novel concept of ‘energy internet’ will rely heavy on the flexibility, performance and reliability of HESS [113, 114].
6 Conclusion
A review of the battery-supercapacitor HESS in standalone micro-grid was presented in this paper. The existing HESS topologies are categorised into three main groups, which are passive HESS, semi-active HESS and full active HESS. Their corresponding characteristics, strengths, weaknesses and possible applications were discussed and compared. The availability of actively controlled components in semi and full active HESS has enabled the use of EMS to manage the power exchange within the HESS. Battery stress can be reduced while maintaining high level of power quality and reliability. A case study was presented to demonstrate the effectiveness of EMS in HESS in mitigating battery stress. The limitations and future trends in the research and development of HESS is analysed and discussed.
