EV PLM – One of these things is not like the other.
Pop Quiz – The figure shows three different charge cycles: an unmanaged charge cycle plus 2 different peak load management (PLM) strategies – sequential and simultaneous – for charging electric vehicles (EVs). The sequential strategy charges the vehicles one at a time, and the simultaneous strategy reduces delivered kW by 50%. Both PLM strategies result in a peak reduction of 50% and take the same amount of time to complete.
Question: Effectively, are both electric vehicle peak load management (EV PLM) strategies the same? If not, how do they differ?
For an energy manager, this is easy, right? Both strategies yield same kW over the same time period which means that they both result in the same peak power and energy use (kWh). Both will result in an identical billing from the electric utility. Therefore, they are the same . . . from a utility perspective. At least that’s how the energy manager will see it. However, the energy manager is neither driving the vehicle nor responsible to have it ready for use. As discussed in a previous post, ignoring these factors can lead to a failed implementation for EV PLM.
To solve this question, let’s look at this question from the perspective of the drivers, the dispatcher and the vehicles.
The drivers view:
In the unmanaged example both drivers arrive at the same time and plug-in. They both see that their vehicles start charging. Our drivers are smart; they note the state-of-charge (SOC) of their vehicles and calculate how long it will take their vehicle to charge. They return at the same time, and they’re both charged and ready to go.
In the simultaneous PLM example, everything seems the same at the outset. Both drivers plug-in and see charging has started. However, when they return each is surprised to find that their EV has only received ½ of the charge that they expected.
In the sequential PLM example, the driver for EV 01 sees no difference. However the driver of EV 02 notices something is immediately wrong – her vehicle doesn’t start charging, and even worse, when she returns with her colleague, her vehicle still hasn’t charged.
This is why most of the consumer facing PLM management systems utilize some variation of the simultaneous strategy. Every vehicle gets some amount of power, but in order to control the load the amount of power scales down depending on the number of vehicles connected. As most consumers top their EV up as often as possible, this is satisfactory as long as their charge level increases some before they’re ready to go. However, for those few EV pioneers that are trying to make a long distance trip, they won’t be happy since they’ll have to wait longer before being ready to depart.
The dispatcher’s view:
In the unmanaged example, both of the vehicles are ready to go at the earliest time. Clearly, this is a dispatcher’s favorite. However, the peak kW charge is twice as high so the energy manager isn’t happy at all. The operating cost for the fleet is higher than necessary.
The simultaneous method allows each vehicle to receive a charge, and if the next route departure isn’t scheduled to depart shortly, there’s no problem. However, if a vehicle is needed quickly, the simultaneous PLM strategy means that neither EV is ready as quickly.
The sequential method has one vehicle ready to go as quickly as the unmanaged example, but the second vehicle won’t be ready for as long as when unmanaged.
My experience with delivery fleets has shown me that in reality, vehicles come and go throughout the day at various times. They seldom arrive simultaneously. A vehicle that returns early often hasn’t used much of its battery capacity and charges quickly. Using the sequential method satisfies the dispatcher more because he can have at least one vehicle ready to depart asap. (For effective use, the control system should allow the dispatcher to prioritize vehicles easily.) For rapid turn fleets such as airport shuttles, the sequential method is far superior since it limits the load yet gets vehicles ready to depart as soon as possible.
The vehicle’s view: (I’m not talking AI here. I know vehicles don’t have views . . . yet. )
The vehicle’s view has to do with parasitic loads (electrical consumption in addition to charging). Some vehicles have to heat or cool their batteries to maintain performance. Others need to keep cargo or cabin temperate. (Thermal pre-heat / pre-cool will be the subject of a future post.) For these vehicles, there’s a huge difference between the two options. In the sequential method, if EV 02 has to wait to receive any power it will continue to use power to maintain parasitic loads while waiting to charge. The delay may result in over / under temperature situations and potential damage to batteries.
This need not be a barrier problem for fleet charging depending on the EV supply equipment (EVSE) chosen. Newer chargers such as the Clipper Creek HCS series charger offer the option to have 4 levels of charging to enable the controls to provide up to 4 levels of charging to allow the system to easily offset parasitic consumption.
And the winner is:
As you can see, I don’t believe that these methods are equivalent beyond the power meter, and in my opinion there isn’t a single clear answer as to which is best. It’s important to match the method of PLM to the user, the vehicles’ intended use, the battery technology and the utility tariff. The above is a highly simplified example. Future posts will explore some of the more interesting complexities.
This post is part of a series explaining how fleet electric vehicle operators can save money charging their EVs. I’m breaking the concepts down into smaller pieces to introduce them to both new and experienced energy managers. For some, these concepts will seem overly simplistic, but I hope to offer easy to understand pieces to help all readers. In upcoming posts I’ll discuss different methods of EV Peak Load Management (“PLM”), operator specific needs and wants, impact of battery chemistry, Demand Response and how Control Dynamix tool, EvAuto, set control the cost of EV charging.