By Mike Heumann and Joseph Gottlieb
For electric vehicle (EV) fleet operators, especially those operating medium- and heavy-duty (M/HD) EVs, EV Support Equipment (EVSE) approaches using remote dispensers (those where the dispenser is physically separate from the AC/DC conversion and power conditioning electronics) are extremely popular. Remote dispensers allow fleet operators to maximize vehicle density in vehicle lots by minimizing the EVSE footprint near the vehicles, while clustering the Power Control Systems (PCSs – the AC/DC conversion and power conditioning electronics) near the electric power inflow location at vehicle lot. However, there are a number of different architectures for these deployments, affecting the ratio of dispensers to PCSs, and the way that the dispensers and PCSs are connected to each other.
Why Remote Dispensers are Popular
For fleets of any sort, maximizing the number of vehicles that can be fit into a vehicle yard is critical. This is especially true for medium- and heavy-duty (M/HD) vehicles such as public transit buses, school buses, and municipal vehicles. These yards often have one hundred vehicles spread over a couple of acres. It is typical to see these vehicles parked end-to-end with little more than a foot of space on the sides between them to maximize parking density. Integrated high-power AC/DC dispensers have fairly large footprints (think the size of a commercial freezer), and putting an integrated charger next to M/HD vehicles significantly impacts vehicle parking density. This is especially true when you considering the need to protect the integrated chargers with bollards or similar barriers. In contrast, the typical remote dispenser has a footprint of roughly 1-2 ft2 (with many less than this), allowing them to be placed in the space on the side of the buses. This eliminates any impact on vehicle lot parking density.
Remote Dispenser Architectures
There are typically three approaches that are used for remote dispenser deployments:
The primary advantage of the serial approach over the parallel approach is that it significantly reduces the number of trenches that need to be dug and cable that needs to be laid during installation. At prices that can reach $10,000-$20,000 per 1000 feet of trenching, there can be significant cost savings from the serial approach.
In both parallel and serial dispenser approaches today, only a single remote dispenser is generally powered at a time. In the parallel case, this is typically accomplished through “1 to X” high-voltage DC switching circuitry, which is either incorporated into the PCS or is located next to it (“X” being the number of remote dispensers per PCS). In the serial case, a two-way high-voltage DC switch is incorporated into each dispenser – it either switches the power from the PCS to the dispenser port (and into the vehicle), or sends it downstream to the next dispenser. While it is possible to have multiple remote dispensers powered at the same time from a single PCS, this requires isolation circuitry to be incorporated into each dispenser to isolate the vehicles from each other. If the battery voltages of the vehicles are different, the dispensers would also have to have DC-DC voltage stepdown capabilities. Both of these features would significantly increase the cost of the dispensers, and the entire system would still be limited by the power capacity of the PCS.
Which of these approaches makes sense for your EVSE depends on your use case. For situations where the amount of charge that each vehicle requires approaches the total power the PCS can output during the vehicle charging window, a dedicated approach is best – the savings achieved by buying fewer PCSs will likely be offset by the impact of possibly not charging all of your vehicles completely. On the other hand, if the amount of charge required by each vehicle is significantly less than the amount of power that your PCS can output during the charging window (say 33 percent or less), then a multiple dispensers per PCS approach probably makes sense. Let’s look at two examples to illustrate when each approach makes more sense:
Of course, most vehicle yards are likely to have a variety of vehicles, each with different energy usage profiles. It is entirely possible that a single vehicle yard could use multiple architectures to maximize the EVSE investment. Weighing the tradeoffs for your particular situation is the best way to make the most of your time and money.
Mike Heumann is VP of Marketing, and Joseph Gottlieb is CTO at Rhombus Energy Solutions. Rhombus Energy Solutions designs and develops products for electric vehicle DC fast charging, electrical energy conversion, and energy management system software.
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