The rise of eMobility brings challenges around how to manage the power load in cities and towns where grid constraints are an issue. This has large impacts for operators of depots both in terms of operating costs and capital expenditure. Load management is an essential tool for a depot operator and can be used to reduce his energy/power costs, reduce the costs of grid connection and reduce delays in the construction phase
What are the challenges for a depot?
A depot for eMobility is typically where electric vehicles return after finishing their mission in order to be charged. This typically takes place in a large hall filled with multiple EV chargers and the charging is done overnight – hence the common term of overnight charging. Depending on the type of vehicles these chargers are either high power DC chargers or lower power AC chargers. Typically, large vehicles such as trucks and buses have corresponding large batteries and use DC chargers whilst light commercial vehicles and cars with smaller batteries use AC chargers. In order to size the problem consider a large inner city bus depot with maybe 200 buses. Typically, eBus’s have batteries in the range of 300 – 450kWh in energy content with a maximum power of around 100kW. Some back of the napkin maths shows that this looks like a potential energy usage of 60 – 90 MWh of energy being consumed. This is compounded by the fact that in most cities there is some form of time of use tariff for electricity – this means that electricity is cheaper at certain times in the day. So, it becomes very important that the depot only uses that energy when it is the cheapest. Even more of a problem would be if we wanted to charge all these buses at the same time (without Smart Charging) because then we get upwards of 20MW in required power from the grid. This represents an enormous investment in power infrastructure (grid connections, transformers, switch gear). With such reasoning, it is clear that we need a means of controlling charging, so we can reduce the power demands and use electricity at the cheapest times. This brings us to load management using Smart Charging.
Load Management for reducing grid connection cost and time
If we want to reduce grid connection cost and time then we will need to use a technique called load shaving or load sharing. Consider the power demand for a typical large depot as shown below. Typically, the vehicles leave and return to the depot in waves. Without smart charging when the vehicles return to base they will start charging as soon as they are plugged in. This leads to large peaks in the power demand corresponding to the return to base times of the different waves. However, with Smart Charging we can “smooth” or “shave” those peaks so reducing the overall power requirements for the depot. The energy delivered stays the same, but we control the charging to spread this out more equally over the night.
Load Management for reducing energy and power costs
Its also possible to reduce the energy and/or power costs by employing a technique called load shifting. Increasingly electricity is being billed using Time of Use tariffs or demand charges. Time of Use tariff is where electricity has a different price according to when it is consumed. A typical city tariff would be where electricity is cheaper between midnight and 6am., more expensive elsewhere. More sophisticated tariff structures are also being seen where electricity prices are structured into different bands throughout the day, i.e. expensive between 7-9am, 11-1pm and 4pm – 9pm. “Demand charges” are similar but are focussed around power rather than energy, but the concept is the same, power is cheaper at certain times in the day, more expensive at others. Taking the typical depot again and overlaying a simple Time of Use tariff onto it highlights the issue facing many depots. Uncontrolled charging means that electricity is being consumed when it is expensive. “Load shifting” with Smart Charging is used to control the chargers so that when the vehicles are plugged in they don’t start charging when the electricity is expensive. Instead, the charging is delayed until after midnight when the electricity prices are lower.
How does Load Management with Smart Charging work?
In Smart Charging we group chargers together into “control groups”. We can then assign a control group a maximum power and then initiate Smart Charging. This maximum power can be a function of time, so we can “shift” power demand to different times. Then with Smart Charging we will control the chargers within the group so that the assigned power limit isn’t exceeded. This is done with some prioritization rules such as First-In-First-Out which prioritizes the vehicles which are first connected.
Some different types of load management – First-In-First-Out.
Let’s take a detailed look at one of the prioritization algorithms First-In-First-Out which aims to prioritize charging for the first vehicles to be plugged in. This is typically used when there is separate entrance and exit for the depot. Considering the simple depot below which holds 9 vehicles. These enter the depot in sequence 1-9 and are parked as shown. Now we can see how the algorithm operates. We assume here that each vehicle needs 4 time blocks at 100kW to fully charge. The maximum power to the depot is 300kW. When the first vehicle returns to base it starts charging, followed by the 2nd and 3rd vehicle. This puts the depot power at the maximum 300kW. Now when the 4th vehicle returns and is plugged in, there is not enough available power to charge all 4 vehicles. So because we are in First-In-First-Out mode we prioritize the first 3 vehicles, so these are charged whilst the 4 vehicle waits. Next the 5th vehicle returns and the 1st vehicle has finished charging. This means we can now start to charge the 4th vehicle, but the 5th vehicle will be waiting. This process goes on through the night, the first vehicles in are the first charged, the later vehicles are charged later and the power constraint is always obeyed.
Some different types of load management – First-In-Last-Out
First-In-Last-Out is typically used in depots where there is a single combined entrance/exit. This means that the first vehicles which come into the depot and park will be blocked by later vehicles arriving. This means that the later vehicles will need to be prioritized before the earlier vehicles. Consider what happens in the picture below. Again we have a simple depot with 9 vehicles. As the vehicles return to base they are plugged in. Similar to the previous case the first 3 vehicles plug-in and start charging, but then we reach the maximum power for the depot. When the 4th vehicle is plugged in because we are prioritizing according to First-In-First-Out we stop charging vehicle 1 and instead start charging vehicle 4. When the 5th plugs in we stop charging vehicle 2, leave vehicle 1 stopped and start charging vehicle 5 (along with 4 and 3). In this way the later vehicles are charged first.
It can be seen that load management through Smart Charging is a key tool for any depot operator. It can be used to reduce both capital expenditure and operating costs which is essential for the long term success for eMobility depots business case. Siemens has developed a number of digital solutions for charge management in the depot. Further details of these solutions can be found here – eDepots or for our friends in North America here www.usa.siemens.com/managingevchargers