Topologies for energy storage in PV systems



Due to recent changes in regulations and standards, energy storage is expected to become an increasingly interesting addition for photovoltaic (PV) installations, especially for systems below 30 kW. A variety of circuit topologies can be used for the battery charger stage, which will require different numbers of semiconductors, voltage classes of the power devices, and in some cases the use of a transformer.

Among the decisive factors for the circuit topology are the battery’s electrical parameters and the required isolation between the battery bank and the inverter. This article describes possible circuit configurations and presents the best matching power semiconductor devices in both, discrete and module forms, to achieve highly efficient and compact systems. In addition, it also discusses the battery technologies expected to be implemented in such storage systems, presenting their main advantages and drawbacks.

Several countries in the world have adopted PV energy in their electrical generation matrix recently. This fact has been boosted majorly by the price decrease of the main components of a PV system, namely the photovoltaic modules and the PV inverter, combined with governmental programmes such as in Germany and Italy, where new PV plants have been subsidised. Financing was done with the main intention of boosting non-polluting, renewable energy. A growing number of decentralised energy sources were an additional consequence.

Fig. 1: Contribution of PV power in Germany on a typical sunny day [1]. Peak generation storage and reuse [2].

Fig. 1 illustrates the current situation in Germany with about 30 GW of PV power installed. The figure presents the contribution of each energy source to the total grid power in a day of high solar irradiation and moderate temperatures. This represents the optimal conditions for PV cells. It can be observed how the installed PV plants cause an energy generation peak around midday.

To avoid disturbances within the grid, the energy demand needs to follow the same profile. Alternatively, the remaining energy sources must reduce their production accordingly, returning to their normal values in the evening, when PV production decreases.

With the increase of PV systems, most of which are grid-connected, a natural consequence is the influence of solar irradiation on the amount of energy injected to the grid. This requires additional considerations within grid management to avoid the fluctuation of frequency and voltage.

Energy storage implementation

A typical PV system with PV modules connected in series and/or parallel is connected to a DC/AC inverter which converts the DC to the AC grid voltage. To store the energy coming from the PV modules, a charge controller stage must be added to the system. This stage is responsible for the correct charging of the battery bank, as well as for recovering the stored energy and feeding it back into the DC line.

Fig. 2: Non-isolated energy storage for photovoltaic systems.

Non-isolated charge controllers

A simple way to implement an energy storage system for PV plants is shown in Fig. 2. A single-phase PV inverter consists of a booster stage followed by a full-bridge inverter. Tied to the DC line, is a charger stage, composed of two switches, two diodes and a filter inductor connected to the battery bank.

The voltage level of the DC line is kept constant by the booster stage and is expected to be higher than the voltage of the battery bank. Common DC line values for single-phase systems are between 360 and 480 V, while the voltage of the battery bank composed of several series-connected batteries is typically between 150 to 250 V.

Therefore, the controller stage will work as a step-down converter when charging the battery bank and as a step-up converter when transferring the energy from the batteries back into the DC line. The configuration in Fig. 2 uses a small number of semiconductors. The circuit is highly efficiency in both charging and discharging paths.

Fig. 3 presents calculated efficiency values as a function of the charging current using a 50 A, 650 V IGBT device [3]. A DC line voltage of 400 V and a battery voltage of 150 V have been assumed. As can be seen, efficiency levels close to 98% during the battery charge at switching frequency of 20 kHz are possible.

Fig. 3: Efficiency results of a non-isolated charge controller based on IKW50N65H5 devices.

At 40 kHz, an efficiency of almost 97% is achievable. For the complete charge and discharge cycle, the resulting efficiency is obtained by multiplying charge and discharge efficiency. This results in a total efficiency of 96% at 20 kHz and 94% at 40 kHz, not considering the batteries’ resistive losses.

In the case of three-phase systems, due to the fact that the DC line voltage can exceed 800 V, power switches with a blocking voltage of 1200 V are required. A power module containing a half-bridge based on JFET technology can be used in a step-up/down topology allowing for bidirectional energy transfer.

The module adds value to the power design by using the SiC JFET channel during the freewheeling period. This enhancement can be achieved thanks to the bidirectional conduction capability of the SiC JFET. Based on measurements, Fig. 4 illustrates the predicted efficiency for a simulated 5 kW three-phase storage inverter.

The efficiency achieved using the power module is slightly higher than the one achieved using JFET combined with a SiC Schottky diode. Semiconductor losses are reduced by about 10%. In both cases, the efficiency for a 40 kHz switching frequency and 650 V DC line voltage exceeds 99% for almost all points of operation.

Fig. 4: Efficiency results of a half-bridge topology with 1200V CoolSiC JFET power module.

Non-isolated topologies provide no galvanic isolation between the battery bank, the PV modules and the grid. For this reason, extra circuitry is recommended to protect the battery bank from any overvoltage coming from the grid, such as from lightning.

This can be implemented by fuses connected to the positive and/or negative terminal of the battery bank, fast enough to disconnect it from the system. Given the typical DC line voltage in both single- and three-phase systems, the non-isolated charge controllers require high-voltage batteries of 150 to 400 V. The use of low voltage batteries would be technically feasible but the suboptimal modulation factor of the switches results in much lower system efficiency.

Isolated charge controllers

To overcome the drawbacks of non-isolated charge controllers, a converter with a transformer providing an intrinsic galvanic isolation between the batteries and the other stages can be used. Moreover, by setting a suitable transformer ratio, it is possible to use low voltage batteries in the range of 12 to 96 V.

Due to the increased number of parts (semiconductors and magnetic components), the isolated solution is expected to be more expensive and less efficient than the non-isolated charge controller. However, by using the latest IGBT technologies in association with soft-switching techniques, it is possible to reduce the semiconductor losses and thus obtain efficiency levels exceeding 95%.

Fig. 5: Schematic (left) and measured efficiency results (right) of a ZVS isolated charge controller, with and without the insertion of a 1 nF capacitor in parallel to the low-side IGBTs.

The left part of Fig. 5 features the basic schematic of a zero-voltage switching (ZVS) converter, where the two legs are driven using a phase-shift modulation technique. A similar converter is described in [5], including details of the modulation strategy. The efficiency curve of a ZVS phase-shift converter is shown on the right-hand side of Fig. 5. The use of additional output capacitors on the low-side IGBTs enables up to 0,8% efficiency increase of the converter.

The capacitors enable a faster extraction of the minority carriers inside the device during turn-off. This will shorten the tail current time of the IGBT, thus reducing the turn-off losses. Using this ZVS topology, the energy stored in the capacitors is not dissipated but returns to the circuit before the turn-on of the switches [6].

Battery technologies

Various battery technologies have been used in electrical systems as storage components. They differ from each other in terms of chemical and electrical properties. Table 1 summarises the main electrical properties of two battery technologies, namely lead-acid and lithium-ion (Li-ion).

For technical and economic reasons, these are the main technologies which are generally used as storage elements in PV systems. Parameters such as energy/price and charge stability reveal a clear trade-off between cost and lifespan of the batteries. Li-ion batteries are roughly three times more expensive than lead-acid batteries with a comparable capacity.

Table 1: Electrical characteristics of lead-acid and Li-ion batteries. [7]
Lead-Acid Li-Ion (LiFePO4)
Specific energy 30 – 40 Wh/kg 100 – 265 Wh/kg
Energy density 60 – 75 Wh/ ℓ 250 – 730 Wh/ℓ
Specific power 180 W/kg 250 – 340 W/kg
Energy/price 7 Wh/$ 2,5 Wh/$
Price per kWh $ 140 $ 400
Self-discharge rate (per month) 3% – 20% 15% @ 40°C
Cycle stability (80% DoD) 200 – 400 cycles 400 – 1200 cycles
Nominal cell voltage 2,1 V 3,2 V

On the other hand, Li-ion batteries can achieve up to 20 years of operation, in contrast to only five years expected for the lead-acid types. An alternative to increase the lifespan of lead-acid batteries is to oversize the storage capacity of the system. This would avoid a deep discharge of the batteries but would increase the cost of the overall installation.

In energy storage systems already commercially available, the choice for battery technology has developed towards Li-ion [6, 8]. Main factor for this decision is the longer lifespan offered by these batteries. Short battery life in solar systems would lead to several substitutions of the batteries during the operational life of the system, thus increasing the overall cost of ownership. Additionally, in some countries the minimum battery lifespan is a requirement to apply for governmental subsidies. This is currently the case in Germany, where a minimum manufacturer warranty of seven years is demanded [9].

References

[1] V Quaschnig, et al: “Ursachen des 52-GW-Deckels und Folgen für die Anlagenentwicklung von Photovoltaiksystemen”, Bad Staffelstein, 2013.
[2] V Brennig, et al: “Dezentrales Speicherprogramm Markt- und Netzintegration der Photovoltaik”, Bad Staffelstein, 2013.
[3] Infineon Technologies, 650 V TRENCHSTOPTM 5 IGBT http://www.infi neon.com/igbt
[4] http://www.infineon.com/SiC
[5] F Di Domenico and R Mente: “ZVS Phase Shift Full Bridge CFD2 Optimised Design”, Application Note. http://www.infineon.com/cfd2
[6] Deboy, et al: “Circuit configuration for offload switching, switch mode power supply, clocked supply, voltage regulator, lamp switch, and methods for operating the circuit configuration”, US 20030094857 A1.
[7] Battery Types and Characteristics for HEV, Thermo Analytics, Inc., 2007. Retrieved June 11th, 2010.
[8] Nedap Power Router website, http://powerrouter.com/
[9] M Fuchs: “Storage to the fullest”, article at PV Magazine, August 2013.
[10] Leipziger Institut für Energie: “Berechnung der Speicherkosten und Darstellung Berechnung der Speicherkosten und Darstellung der Wirtschaftlichkeit ausgewählter Batterie-Speichersysteme”

Contact Dirk Venter, Arrow Altech Distribution, Tel 011 925-9666, dventer@arrow.altech.co.za

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