As the world moves away from fossil fuels towards renewable energy, the electrical grid is changing to keep up. The old way of doing things — large coal, gas, or nuclear power plants supplying power to consumers in a monopolistic fashion — is giving way to a smart grid that features a combination of traditional and renewable energy sources from both public and private suppliers. There’s a corresponding rise in the need for bidirectional power supplies to ensure the efficient transfer of power between various smart grid elements. In this blog, we’ll examine bidirectional power supplies, their applications, and how RECOM is helping our customers meet these new demands.
The Power Grid is Changing
The foundation of the traditional power grid dates back to 1935. It’s characterized by a one-way flow: power flows from generating stations through the distribution network to consumers. Generating capacity is relatively static with very little excess capacity available to accommodate sudden spikes in demand.
As a result, the traditional grid suffers from power instability as the system struggles to respond to rapid load shifts. The reliance on old electromechanical control technology provides little control ability or up-to-date information on usage patterns. The one-directional power flow makes it difficult to benefit from renewable energy sources such as wind or solar.
Utility companies are actively working on upgrading the grid. The smart grid aims to provide real-time monitoring and control, integrate wind and solar power sources, and balance energy supply and load demand.
Energy Storage: The Missing Piece of the Puzzle
Increasing the proportion of power sourced from wind and solar also increases uncertainty in power generation capacity. The amount of energy available from the wind and the sun varies in unpredictable ways, so power from renewable energy sources is more variable than power from fossil fuel and nuclear power plants. A typical solar installation supplies less than 25% of its theoretical maximum possible output (Capacity Factor, or CF) over the course of a year. For a wind turbine, it’s less than 40%. Contrast that with the 90%+ CF of a nuclear power plant. In addition, the available capacity of renewables cannot be ramped up to meet the minute-by-minute demand for electricity.
Energy storage provides a method to balance supply and demand. When demand exceeds supply, the storage system provides the extra power needed to stabilize the grid and avoid brownouts or shutdowns. When the supply of electricity exceeds the demand, the surplus generating capacity is used to recharge the storage system.
Energy Storage and Renewable Energy
Pumped-storage hydropower is currently the most widely used energy storage system (ESS) technology, but a battery-based design is the most scalable technology and is showing the highest growth.
Figure 2 shows the main functional blocks in a grid-scale ESS that uses batteries to store energy. Bidirectional power supplies transfer AC power from the grid to the storage system and vice versa. AC power from the grid is converted to DC power to the batteries to charge the storage system; when the storage system is helping stabilize the grid, DC power is converted to AC power and fed back into the grid.
In many cases, the ESS is combined with a renewable energy source. In this case, green energy from the wind turbine (an AC source) or PV array (a DC source) can be directed to the battery array or back to the grid as appropriate. Solar panels mounted on commercial or residential buildings can also supply power to an ESS or back to the grid.
All of this energy flow needs to be controlled, coordinated and monitored. RECOM provides many low power high-isolation DC/DC converters (with up to 20kVDC isolation voltage) for the battery management systems, communication networks, and the various voltage, current, temperature, fire and pressure sensors required to built reliable and safe battery energy storage systems.
EV Charging
Electric vehicles are another growing application for bidirectional power supplies. As EVs operated purely on battery power continue to increase market share, the installed battery capacity per vehicle is also increasing. Consumers are also demanding faster charging times for larger capacity batteries. This demand is spurring an increase in battery operating voltage from 400V to 800V, beginning with high-performance vehicles.
An EV equipped with sufficient battery capacity is potentially capable of acting as an ESS, enabling a variety of use cases: vehicle-to-home (V2H) power generation, vehicle-to-grid (V2G) opportunities, vehicle-to-vehicle (V2V) charging, or jumpstarting another EV. Current EV charging stations and EV onboard chargers (OBC) are unidirectional systems, but these new use cases are driving a transition to a bidirectional infrastructure.
Scenarios that call for bidirectional power supplies in EVs and EV charging stations include:
- EV supplying power back to the grid or to a microgrid in the home.
- EV charging station supplying power to an EV either from the grid or from stored energy depending on relative electricity prices.
- EV charging station recharging an onsite battery installation.
Unidirectional vs Bidirectional Power Architectures
A bidirectional power supply demands a different design approach compared to an equivalent unidirectional supply. A unidirectional AC/DC power supply designed for high efficiency uses wide bandgap (WBG) SiC or GaN power devices with a totem-pole power factor correction (PFC) front end driving a DC/DC topology such as an LLC resonant converter.
Although the totem-pole PFC topology is bidirectional, the resonant LLC is not. For bidirectional applications, a CLLC resonant converter is preferred for the DC/DC stage, as it combines high efficiency with a wide output voltage range in both charging and discharging modes. The CLLC employs soft switching to maximize efficiency: zero voltage switching (ZVS) on the primary side, and ZVS combined with zero current switching (ZCS) on the secondary side. SiC transistors are fast becoming the dominant technology in such high-power applications.
Figure 3 shows an example of a bidirectional power stage for a three-phase charging application. This design requires 14 power transistors. It is an isolated topology, so the power transistors are paired with isolated gate drivers and isolated DC/DC power supplies.
RECOM’s Resources for Bidirectional Power Supplies
RECOM can supply high-reliability custom battery chargers, conditioners, and bidirectional inverters based on proven platform designs from three-phase AC supplies with power ratings of up to 30kW or even higher with multiple units connected in parallel.
RECOM also has an extensive library of platform designs to call on via our sister company PCS, who specializes in fast turnaround, high-power, and full-custom solutions. These proven designs can often be readily adapted for other applications without necessarily incurring extensive development or certification costs.
Conclusion
The advent of the smart grid and the rise of renewable energy are leading to an increased demand for bidirectional power supplies that transfer AC or DC power between energy sources, energy consumers, and storage systems. RECOM is involved in every element of the smart grid, from low-power DC/DC inverters used to isolate battery management systems or wind turbine controllers, to low standby consumption AC/DC modules to power smart meters, EV chargers, and PV inverters, to kilowatt-scale converters for off-grid, ESS, and other applications. Ready to get started on your next design?
