The effect of the hysteresis band on power management strategies in a stand-alone power system
Introduction
Energy production by renewable energy systems protects the environment from further deterioration due to the global warming effect. Solar and wind energy are abundant, free, clean, and inexhaustible. Other advantages of photovoltaic systems (PV-systems) and wind generators include the long lifetime and low maintenance requirements [1]. Electrification of remote places such as small islands and rural areas, powering of telecommunication stations, and the energy-demanding desalination of water are a few of the numerous applications in which renewable energy sources (RES) are being exploited. Combinations of PV-systems and wind generators with energy storage system for the surplus energy have widespread use around the world [2], [3], [4], [5], [6], [7], [8], [9]. Most cost-effectiveness studies so far show that current technologies for power supply such as diesel generators are advantageous for small applications [10]. However, disadvantages of diesel generators include greenhouse emissions, high maintenance costs, and problematic capacity expansion to meet future demands in remote areas. On the contrary, RES stand-alone power systems with hydrogen storage ensure eco-friendly operation, and are easily expandable with relatively low maintenance costs and therefore attractive candidates for small- and medium-scale applications.
Hydrogen constitutes a promising form of alternative fuel for the future, but the lack of a safe and cost-effective hydrogen storage system is currently a significant obstacle to its direct use in a fuel cell. The most eco-friendly process for hydrogen production is by water electrolysis, where no harmful emissions are generated. The decomposition of water, though, requires large amounts of electrical energy that only RES can provide in an inexpensive and environmentally friendly way.
The main components of the stand-alone power system under study include a PV-array, a set of wind generators, a short-term storage system based on lead–acid accumulators, and a long-term storage system that consists of an electrolyzer, a fuel cell, and a hydrogen storage system with pressurized tanks.
The mathematical models of all these subsystems that describe the operational behavior in steady-state or dynamic mode have been presented and analyzed in the literature [11], [12], [13], [14], [15], [16], [17], [18], [19]. The experience gained from the operation of different stand-alone power systems across the world [20], [21], [22], [23] is a valuable resource for the selection of proper operating policies of similar systems. Design of such systems is also an issue that has attracted significant attention. Generally, solar and wind energy production capacity play a major role in the selection of proper sizes for all units in integrated power systems [24], [25], [26], [27], [28]. Optimization strategies based on cost minimization of the integrated system utilizing a short-term and a long-term storage system can prove quite efficient [29], [30], [31], [32], [33], while various proposed power management algorithms have been evaluated towards reliable power supply [34], [35], [36]. The main gist of most power management strategies (PMSs) is that above a maximum limit for the accumulator state of charge (SOC), the electrolyzer may operate by power provided by RES units occasionally assisted by the accumulator and below a minimum limit for the accumulator SOC; the operation of the fuel cell meets the load demand.
Most of the current studies, however, select some key variables of the system such as the SOC limits arbitrarily without delving into their effect on the overall system performance. Ulleberg [32] used an operation strategy with a broad hysteresis band size where the main performance factor was the stored hydrogen at the end of the simulation period. The use of a hysteresis band for the electrolyzer and fuel cell resulted in prolonged operation for the fuel cell in some case studies. Ghosh [35] proposed several operation strategies but used solely the stored hydrogen as performance indicator. Previous work of our research group [36] proposed three PMSs without hysteresis band and evaluated their performance using a simulated model of the integrated system. Through sensitivity analysis the key operating parameters were identified and the impact on performance was evaluated. More specifically, the minimum limit for the SOC of the accumulator, SOCmin, which defines the depth of discharge (DOD) of the accumulator and the output power of the fuel cell, affected the system operation significantly in terms of utilization for the accumulator. The main outcome was that unnecessary heavy utilization of individual units reduced the unit lifetime and further resulted in irregular operating pattern with frequent ignition and termination instances for the electrolyzer and the fuel cell.
In the present work, the hysteresis band on the SOC limits of the accumulator is introduced in two PMSs that were proposed in [36]. In those strategies, the SOC of the accumulator was held within two limits only: the minimum SOCmin, where the fuel cell would operate in case of shortage of power and the maximum SOCmax, where the electrolyzer would operate in the case of excess of power. The hysteresis band is defined as the zone around the boundaries of the SOC limits of the accumulator, where the electrolyzer and the fuel cell are allowed to operate in cases of prolonged excess or shortage of power, respectively. Such an operating policy safeguards the electrolyzer and the fuel cell from frequent start-ups and shut-downs. In addition, the advantages of the hysteresis band in the PMS include the protection of the accumulator from heavy utilization. The key characteristics of the hysteresis band, such as the limits that determine the bounds of operation for the various subsystems, will be calculated as part of the development of a reliable and effective PMS.
The structure of the paper is as follows: In Section 2 the various subsystems are briefly described with reference to the mathematical model used for the simulation studies. Section 3 introduces the proposed PMSs with the hysteresis band for the integrated system. Section 4 reports the simulated results and evaluates the performance of each PMS towards certain criteria. In addition, a sensitivity analysis of the system performance with respect to key decision parameters attempts to identify the optimal parameter values for the proposed PMSs.
Section snippets
Description of the stand-alone power system
A 1 kW application utilizing solar and wind energy with hydrogen production through water electrolysis, storage in pressurized tanks, and subsequent utilization in a fuel cell is currently under development at Neo Olvio of Xanthi in Greece. The renewable energy system consists of a PV-array and three wind generators. Surplus energy can be supplied to a polymer electrolyte membrane (PEM) electrolyzer for hydrogen production. Hydrogen is then stored in pressurized cylinders for subsequent use in a
Power management strategies
The main objectives of the proposed PMSs for the autonomous stand-alone power system are the reliable load requirement satisfaction and the protection of the subsystems from operating outside the range of desirable conditions. The PV-array and the wind generators produce power, which is basically used to meet the 1 kW constant load demand. Any surplus of energy can be potentially stored in the form of hydrogen through water electrolysis and any shortage of power can be met by the accumulator or
Simulated results
The performance of the stand-alone power system under the two proposed PMSs over a typical 4-month time period is calculated. SOCnom=100% is the accumulator SOC when fully charged. SOCmin and SOCmax are selected at 84% and 91%, respectively. Accumulator manufacturers recommend that these two boundaries, SOCmin and SOCmax, should not be severely violated. A total of three alternative PMSs with different operating parameters have been tested. Case (a) refers to the hysteresis band range equal to
Parametric sensitivity studies
The use of a very low SOCmin in a system with constant charges and discharges would eventually result in the early replacement of the lead–acid accumulator due to its intensive use. Lead–acid accumulator manufacturers recommend that a very low value for SOCmin and consequently a high value of DOD should be avoided in order to prolong the lifetime of the accumulator. In a previous work [36], an investigation of the effect of different values of SOCmin on the system performance concluded that low
Conclusions
The present study introduced the idea of the hysteresis band into two different PMSs for a stand-alone power system using solar and wind energy with hydrogen production as energy storage. The decision variables in the PMS are the net power from the RES after meeting the load demand and the accumulator SOC. Accumulator SOCmin and SOCmax levels determined the operation of the fuel cell and the electrolyzer, respectively. A hysteresis band was used for the operation of the fuel cell and the
Acknowledgements
The financial support of the European Fund of Regional Growth and the Region of Eastern Macedonia and Thrace with the General Secretariat of Research and Technology as the final beneficiary under project contract (ΠEΠ/AMΘ 9) in the operating project of Eastern Macedonia and Thrace is gratefully acknowledged.
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