Because of the rising demand for readily available electricity, communities are striving to increase their usage of renewable energy, frequently in a synergistic relationship with both existing and emerging Energy Storage Systems (ESS) technologies. Wind and solar power combined with lithium-ion battery installations is one of the fastest-growing solutions with ESS as an essential part of these systems.

The market outlook

As of March 2019, the two largest operating utility-scale battery storage sites in the United States (US) provide 40 MW of power capacity each: Golden Valley Electric Association’s battery energy storage system in Alaska and Vista Energy storage system in California

In the US, there are 16 operating battery storage sites which have an installed power capacity of 20 MW or more. Of the 899 MW of installed operating battery storage reported by states as of March 2019, California, Illinois, and Texas account for a little less than half of that storage capacity. California leads the pack by far with 230 MW. See Figure 1, at right.

In January 2020 the U.S. Secretary of Energy Dan Brouillette announced a comprehensive program to accelerate the development of next-generation energy storage technologies that would position the U.S. as a global leader.

In 2020, Lux Research reports that the global energy storage market is expected to grow to $546 billion by the year 2035. One of the largest markets will likely be residential energy storage, with an expected compound annual growth rate of 76% and $8 billion revenue increase over the next three years.

Figure 1

Figure 1: US utility-scale battery storage power capacity in MW (Image from US Energy Information Administration)

Why do we need energy storage?

Energy consumers are fast becoming active power producers while also demanding clean, reliable, and affordable power.

The changes to modern energy generation and consumption is being driven by three powerful trends: the more affordable distributed power technologies, decarbonization of the world’s electricity network via the integration of more renewable energy sources, and the emergence of digital technologies.

Battery energy storage solutions offer new application flexibility and open new business value throughout the energy value chain, from conventional power generation, transmission and distribution, and renewable power to industrial and commercial sectors. Energy storage enables diverse applications including firming renewable production, stabilizing the electrical grid, controlling energy flow, optimizing asset operation and creating new revenue.

For renewables developers, energy storage offers a faster alternative to a Power Purchase Agreement (PPA), which may have a lead time of a year or more. For utilities, energy storage offers relevancy with increased distributed generation. Energy storage can help renewables developers increase the dispatchability and predictability of renewables, helping to meet strict code and connection permits.

Using traditional Si power devices in ESS

Designing high power bidirectional inverters with silicon (Si) power transistors will lower the system power performance with high conduction and switching losses. Efficiency will suffer, more cooling will be needed, and more board space will be taken as opposed to using Wide Bandgap (WBG) devices, such as Gallium Nitride (GaN) or Silicon Carbide (SiC).

Si lower operating frequencies will necessitate larger filtering inductors and capacitors; this will lead to increased cost, weight, and volume in the design. Increased inductor losses will also reduce efficiency.

WBG power devices provide the best energy storage solutions

GaN power transistors

A good example of a distributed energy storage device (DESD) incorporates an isolated bidirectional DC-DC converter with 650V GaN transistors. The DESD integrates a 13.2V low-voltage Li-ion battery pack, an embedded bi-directional DC-DC converter and wireless communication system. These three parts can be packaged together, enabling it to be directly connected to high-voltage (380V) DC grid. This enables a modular approach for battery energy storage systems. Two 650V enhancement mode GaN transistors are used at the high voltage side. A 400V to 12V DC (for example, auxiliary power), 1kW converter for 1kWh DESD is shown in Figure 2, at right.

GaN power devices enable a marked improvement over a Si device. GaN transistors make performance improvements by expanding the operation range to light load, reducing switching loss and EMI, increasing the total efficiency of charging and discharging operation.

A half bridge center-tap design, with an active clamp power stage, can be designed to provide considerable benefits in low voltage and high current bidirectional power conversion. See Figure 3, at right.

GaN power switches will also extend the safe operation area (SOA) and increase efficiency over Si devices. GaN devices achieve this improved performance over Si devices due to the smaller COSS capacitance which enables lower turn-off losses; this can allow the converter to operate in a hard switching mode. Also, there is near zero reverse recovery time and reverse recovery charge of the body diode in a GaN device that shortens the turn-off time when the high voltage side switches operate in the rectifying mode.

SiC power transistors

Silicon carbide (SiC) technology provides another excellent power improvement over Si at the core of ESS solutions. SiC power semiconductor solutions can enable at least 50% more efficiency than Si and will easily handle higher grid-scale voltages. The system-level gains added performance from the efficiency, power density, and fast switching speed that SiC power devices provide.

Using a SiC module: In order to reach high switching speeds with low switching losses, a package can be designed to achieve low stray inductance for both the module and the system-level busbar design.

The current loops within Wolfspeed modules are wide, low profile, and yield even distribution between the devices, resulting in equivalent impedances across a switch position. The power terminals on the module are also vertically offset. This enables design of simple bussing between the DC link capacitors and the module, laminated all the way up to the module without requiring bends, coining, standoffs or any complex isolation. The result is a power loop stray inductance of just 6.7 nH at 10 MHz — as demonstrated in the XM3 inverter reference design. See Figure 4, at right.

With half the weight and volume of a standard 62 mm module, Wolfspeed’s XM3 power module platform maximizes power density (up to 450A) while minimizing loop inductance and enabling simple power bussing. See Figure 5, at right.

The XM3’s SiC-optimized packaging enables 175 °C continuous junction operation.

Energy storage systems are critical for the fast-growing renewable energy market worldwide. The use of Silicon power devices in such a design does no justice to the size, weight, and power of these systems as compared to using WBG devices which will also greatly improve efficiency and operate at higher voltages.

Figure 2

Figure 2: 380V DC microgrid system design with distributed energy storage devices (DESD) and distributed renewable energy resource (DRER) (Image from Reference 1)

Figure 3

Figure 3: A half-bridge center-tap architecture, using an active clamp converter, employs GaN transistors as the power element on the high voltage side (Image from Reference 1)

Figure 4

Figure 4: A side view of the XM3 module with non-planar power leads shown (Image from Wolfspeed)

Figure 5

Figure 5: A three-phase inverter bussing layout (Image from Wolfspeed)

References
  1. Distributed Energy Storage Device Based On A Novel Bidirectional DC-DC Converter With 650V GaN Transistors, Fei Xue, Ruiyang Yu, Wensong Yu, Alex Q. Huang, FREEDM Systems Center, North Carolina State University, IEEE 2015