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DC/DC Bidirectional Conversion: An Increasingly Important Part of the Energy Storage Puzzle

by epic power

A bidirectional DC/DC (DC-to-DC) converter is a piece of power electronics that can transfer energy in both directions between two DC buses. They are also known as four-quadrant converters because both current and voltage can change direction. Lately, they are referred to as DC transformers. They are represented in figure 1. The main features of bidirectional DC converters are:  

  • power handling capability P
  • voltage values in both sides (v1, v2) and ratio thereof, also known as gain (d=v1v2) and
  • efficiency, a figure that describes how much energy is lost in the conversion. It is an important feature of bidirectional converters because energy goes back and forth, so overall efficiency is the product of the efficiency in each direction (not always equal).

They are easily parallelizable units; no synchronization is needed, so they can be stacked for higher power levels with no additional communication between them.

 

Isolated vs. non-isolated. That is the question.

Bidirectional converters can be isolated and non-isolated. Isolation implies a galvanic separation between the ports, which means there is no direct current path between them. This results in improved safety, sometimes required by regulations. It also results in an improved robustness of the converter because if something goes electrically wrong on one side, the other side remains unaffected. In order to achieve galvanic isolation the use of internal transformers is required (see figure 2). If the voltage transformation ratio (aka gain) is high (d>20), then a transformer is internally required, so in those cases isolation is implied. The use of a transformer requires an internal high frequency dc/ac and a subsequent ac/dc transformation. The term “high frequency”  implies that values can range from several kHz to MHz. The higher the frequency, the smaller the transformer. Of course, it all comes with a cost. Isolated converters are typically more expensive (double €/W ratio or more), so in many occasions the question arises: Do I really need isolation?... and the answer is a typical one: it depends.

Additional practical features of DC/DC converters are the following:

  • cooling modes (air cooled, with or without forced ventilation or liquid cooling),
  • control modes (current control, voltage control) and control signals or buses (CAN bus, CAN Open, Modbus, Profibus),
  • Volumetric power density (W/l) or gravimetric power density (W/kg) and last but not least,
  • dynamics of the converters (how fast they can respond to command changes).

Side features, sometimes-relevant ones, are:

  • capability of selfpowering from one DC bus or the other (or both),
  • softstart capabilities to avoid in-rush currents and
  • very low, ideally zero, minimum controllable current on both sides

Figure 3 shows a 3,5 kW isolated and bidirectional multi-purpose commercial DC/DC.

Applications of bidirectional DC/DC converters

Bidirectional converters have been mentioned in  literature since 1978 when S. Cuk, R. Middlebrook and W. Behen presented a compact switching circuit capable of bidirectional power flow as a single circuit. The work was applied to charge and discharge batteries in spacecraft applications. Since then, bidirectional DC/DC converters are being increasingly used in the following fields:

  • Energy recovery systems based in supercapacitors for elevators, cranes, and other braking applications such as for trains or roller coasters. One of such units, including the supercapacitors, is depicted in figure 4.
  • Electric and hybrid vehicles, to transform energy between different DC buses connecting different types of batteries and also fuel cells.  
  • In intralogistics shuttles, AGVs or other industrial vehicles to supply AC motors from low voltage batteries or supercapacitors.
  • In hybrid boats and hybrid sail boats
  • In UPS systems
  • In smart grids, as DC transformers between different voltage levels. The use of these converters is expanding due to a recent trend of micro and nano grids in DC. 
  • In power train test systems, or in battery test systems to avoid energy losses in the test procedure
  • In peakpower-reducing apparatus in many different fields: elevators and cranes, diesel generators, railway electric distribution   
  • To supply solar tracker motors directly from batteries. These can be directly charged from the solar panel.
  • For Vehicle2-Grid (V2G) applications where the car batteries are part of a smart system
  • In Powerto-Gas applications, to directly energyze electrolyzers from solar panels with MPPT functions integrated in the system. Figure 5 shows a modular converter used for such an application.
  • To hybridize different types of batteries  or batteries and supercapacitors to extend the life expectancy and reduce the overall costs of energy storage systems

In general, they can be used to avoid redundant transformations of DC to AC to afterwards transform AC back to DC. Definitely, something that is of key relevance in the EES fields of interest.

Future trends in bidirectional converters

Quite recent, but already well established, is the use of wide bandgap semiconductors, in particular Silicon Carbide (SiC), as switching semiconductor components inherent to power electronics. This type of semiconductors paves the road to higher frequencies and an extended temperature range of operation. Used in conjunction with soft-switching techniques, efficiencies reported are up to 98-99% (in one direction). Soft-switching occurs when the switches are turned on and off with either a very low voltage or current, thus producing lower losses.

Future trends show potential applications of what is known as partial power processing configurations. Converters cannot only be stacked in parallel but combined as a series of converters with a smaller power handling capability than that of the overall one yields to improved performance and reduced costs in some cases.

Epic power designs, produces and customizes bidirectional DC/DC converters. For more insight and further references, check a more detailed version of this article.

 

Pilar Molina Gaudo

CEO

epic power


The ees International Magazine is specialized on the future-oriented market of electrical energy storage systems, not only from a technological-, but also a financial and application-oriented point-of-view. In cooperation with ees Global, the ees International Magazine informs the energy industry about current progress and the latest market innovations.
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