Power Management of Hybrid Energy Storage System for a MW Photovoltaic System Guishi Wang, Mihai Ciobotaru, Vassilios G. Agelidis Australian Energy Research Institute & School of Electrical Engineering and Telecommunications The University of New South Wales, UNSW Sydney, New South Wales, 2052, Australia E-mails:
[email protected],
[email protected],
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Abstract—The power output of photovoltaic (PV) systems is affected by variable weather conditions, and thus presents a major obstacle to their extensive penetration into the electricity network. This paper proposes a power management strategy for a hybrid energy storage system supporting a 1 MW PV power station. The storage system is based on a vanadium redox battery and a super-capacitor bank. The strategy focuses on improving the efficiency and life span of the vanadium redox battery while smoothing the output power fluctuations of the PV system. The equivalent electrical models of 1 MW PV system and the hybrid storage system have been implemented in MATLAB/Simulink & PLECS software platform. The satisfactory operational performance of the proposed strategy is proven through simulation results. Keywords-Hybrid energy storage, Vanadium redox battery, Super capacitor, Power management, Photovoltaic
I.
INTRODUCTION
The fluctuating output power of large solar photovoltaic (PV) power stations could challenge the reliability and stability of the electricity network. For instance, the unpredicted and oscillating power profile of a large-scale PV power station may cause voltage fluctuations and/or frequency deviations at the point of common coupling (PCC) [1]. Integrating an energy storage system (ESS) with a large PV power station can improve its output power profile [2, 3]. The feasible options for the ESS technologies may include batteries, compressed air, flywheels, super-capacitor (SC) banks and superconducting magnetics [4-6]. The compressed air energy storage (CAES) is ideal for high power applications with large energy capacity. However, this technology depends on the geography of the station`s location. The flywheels and SCs are normally used for applications with short term energy storage and high power ratings. The superconducting magnetic energy storage (SMES) is another option for short term energy storage. The battery energy storage systems (BESS) include a large range of options, such as lead-acid battery, li-ion battery, sodium sulphur (NaS) battery and vanadium redox battery (VRB). The li-ion battery offers great performance in terms of power and energy densities. However, its wide application has been constrained by the high manufacturing cost [4-6]. Also, cost is one of the hurdles for introducing the NaS battery to large PV applications. Moreover, this battery has to operate at
high temperature (300~350 ℃ ), which is another major obstacle of NaS battery [4-6]. Up to now, the lead-acid battery as a mature technology dominates the BESS market due to its relatively low cost. However, such battery may have a short service life, typically less than 1000 full cycles [7]. Compared to conventional batteries, the VRB has a relatively longer service lifespan. The reason is that the VRB has almost no aging issue caused by chemical reasons [8, 9]. Also, the cost of VRB is still high nowadays. Compared with single ESS technologies, the hybrid energy storage system (HESS) can easily satisfy the particular requirements for renewable energy applications, such as high power rating, high energy capacity and short response time. General combinations include CAES and SCs, BESS and SCs, SMES and BESS, and others. In [7], it is shown how the SCs can improve the lifetime of the lead-acid battery in a wind energy system. A BESS&SC system is proposed in [10] to achieve longer battery lifetime and higher overall system efficiency. A power management strategy is presented in [11] to maintain the state of charge (SOC) of both BESS and SMES within specified range. Besides the long lifespan, the VRB naturally offers flexibility in designing its power and energy ratings independently of one another. In other words, the power rating depends on the amount and size of power stacks, while the energy capacity can be easily expanded by increasing the size of electrolyte tanks. These features increase the scalability of the VRB and thus being easy to adapt to various PV system conditions and applications. However, the VRB efficiency is low if the output power is less than the 20% of its rated power [12, 13]. On the other hand, the operating power level of the SCs has little influence on their efficiency. The SCs also offer fast charging time and high number of recharging cycles, and thus the SCs can be used together with the VRB in a complementary manner. This paper proposes a HESS combining the advantages of both VRB and SCs. A power management strategy of the HESS has been developed to support a 1 MW PV power station to provide constant output power during every 5 minutes (in accordance with the Australian Energy Market trading interval [14]). This proposed strategy can also control the operating points of VRB and SC, thus leading to a better efficiency and an improved lifespan. Additionally, the SOCs
II.
100
Efficiency (%)
of VRB and SC are limited in the given range by the proposed strategy. The rest of the paper is organized as follows: Section II describes the equivalent electrical models of the PV system and the HESS including the VRB and SC. In section III, the proposed power management strategy is explained in detail. The simulation results of three case studies are presented and discussed in section IV. Finally, the conclusions of this work are summarised in section V.
For simplification purposes, a MW-level PV system has been modelled as a first order system HPV(s) [15], as given in (1). Pnom PPV k c H PV ( s ) G k c G (1) S s 1 2 a where PPV is the power output of the PV system, G is the solar irradiation provided by a sensor located in the centre of the PV system area, S is the land area of the PV system, a is a constant provided by curve fitting to experimental sets of data in [15], Pnom is the rated power of the PV system and k c is the gain responsible for the power curtailment, ranging between 0 and 1. B. Vanadium Redox Battery An electrical equivalent model of the VRB has been proposed in [16-19]. The power rating of this VRB model can be expanded by connecting several standard stack models in series [12, 20]. A VRB equivalent model consisting of five stacks arranged in series has been used in this study. For a more realistic model, each stack has been considered having slightly different efficiency profile. As shown in Fig. 1, the overall efficiency of the VRB is strongly influenced by the operating power level. The powerefficiency curve has similar characteristics when the VRB works in charging or discharging mode. Specifically, the efficiency increases and then decreases when the output power changes continuously from the rated value to zero. It is worth noting that a significant drop occurs when the output power of the VRB is between zero and 20% of its power rating.
discharging
40
0 −1
−0.5
0 Power (pu)
0.5
1
Fig. 1. Efficiency-power curves of VRB at fixed SOC (0.5 pu).
Fig. 2. Third order Ladder model of SC.
increasing the number of SCs connected in series and parallel. D. PV power station including HESS Fig. 3 presents the integration of the HESS to smoothen the power output of the PV system. The core components of this PV power station include the PV system, the HESS, and the grid-connected inverter. The VRB and the SCs are connected to a common DC bus through two separate bidirectional DCDC converters. Following the power management strategy in the next section, these two converters can control the operation modes of the HESS and the power flow between the VRB and the SC bank. The DC bus voltage is regulated to a constant value by the grid-connected inverter. Vdc
PV Power Station idcPV1
iPV1 VPV1
idcPVn
idcSC
PV System
PV Array n
iSC VSC
idcVRB
PV Array 1
iPVn VPVn
C. Supercapacitor One of the widely used equivalent electrical models of the SC is the Ladder model [21, 22]. The increasing order of the Ladder model induces higher accuracy, but increases the computational time. According to a comparison study of different order Ladder models [21], the 3rd order Ladder model can present the accurate SC characteristics when the simulation time step is around milliseconds range. The 3rd order SC model shown in Fig. 2 is chosen, because the simulation time step of this study is of the order of milliseconds. In order to fulfil the requirement of the output voltage and current, the SC bank has been expanded by
charging 60
20
SYSTEM DESCRIPTION
A. PV System
80
iVRB
SC Bank
VRB-ESS
VVRB Cdc
idc Grid
Fig. 3. Topology of the PV power station including the HESS.
HESS
POWER MANAGEMENT OF HESS
III.
* A. 0.05 pu< PHESS
