Charging and energy management system - hub-satellite vs. open systems: how to find the best charging solution for multiple electric cars

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The Mobility House Team

July 05, 2021

Estimated reading time: 6 minutes

Electric cars are experiencing a boom, a fact confirmed by the new vehicle registration statistics, which show electric cars accounting for 20%. And for good reason: in vehicle fleets, for example, electric cars show their strengths particularly well, as they bring both economic and ecological benefits. For companies, this has a positive impact on both the annual balance sheet and how the company is perceived from the outside. As the company’s electric vehicles are often charged directly on site, fleet managers have to take on various new responsibilities, such as planning and managing suitable charging infrastructure and selecting a proper system for the intelligent control of charging processes. Here, too, the variety of options makes progress clear - be it so-called hub-satellite solutions or open systems.

The same applies for home and property owners: more and more private individuals are also buying electric cars and they would naturally prefer to be able to charge them at their own parking space. For some time now, tenants and owners of condominiums have had a statutory right to a wallbox at home, and accordingly the topic of electric mobility can no longer be avoided.

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The charging and energy management system as the core issue

An efficient charging and energy management system is generally one of the key issues when it comes to setting up charging infrastructure. As peak loads in the energy consumption massively drive up costs because of the higher power price, the charging infrastructure should be able to intelligently control the electricity intake for larger numbers of electric cars. Nevertheless, given the amount of time the cars usually spend in a parking position and the distances they cover, there is no need for all the vehicles to be charged at the same time or at full capacity. Charging electric cars at reduced capacity and with a time offset makes it possible to significantly reduce costs without limiting their availability or infringing on the user’s mobility needs. 

With an intelligent charging and energy management system like ChargePilot® from The Mobility House, both the one-time investment costs and the ongoing energy costs for the mains connection and power charges can be reduced by up to 70%. A brief practical example can highlight the saving potential: a company installs 20 charging stations, each with a capacity of 22 kW. Using all charging points at the same time and at full capacity would require a connected load of 440 kW. With an intelligent charging and energy management system, the load can be significantly reduced—in individual cases by up to 100 kW, less than a quarter compared to uncontrolled charging. In the example given, this would save one-time setup costs of up to 30,000 euros along with up to 27,000 euros a year for the demand charges thanks to the avoidance of higher load peaks. The graph below illustrates the savings:

Graphics: savings by a charging and energy management system

Even more savings can be achieved if the charging times are postponed to cheaper tariff times—for example overnight, when the vehicles are parked anyway. For industrial customers, tariffs that are dynamic and with prices oriented on the spot market can enable even bigger savings. Even electricity produced from a solar plant, ideally in conjunction with stationary storage, can further reduce costs, as charging the electric fleet significantly increases the level of own consumption and the costs for generating power in solar plants are less than ten cents.

Key factors and system options—an overview

If setting up and operating charging points is to be kept as simple as possible, a charging infrastructure tailored to suit multiple electric cars should take into account a whole series of technical and economic factors:

  • the one-time and the recurring costs;
  • the integration into the existing energy environment;
  • cost-optimized control of the individual charging processes, while ensuring the that individual mobility needs are met;
  • dinterfaces for simple billing of the charging processes.

It must be noted, however, that not every system is able to take all factors into account equally. There are basically three different systems for charging infrastructure with more than one charging point:

  • Double-charging stations—that is, a wallbox with two connections—that split a specific power value across the two charging points. These are particularly suitable for detached houses.
  • Closed hub-satellite solutions with the hub wallbox functioning as the central controller, controlling the other attached charging points. Also known as leader-follower system or master/slave. Limited to one manufacturer and currently roughly 10 to 20 charging points. Suitable for smaller charging infrastructure projects that need to be realized at short notice and will not continually grow.
  • Open systems, which can combine a number of different charging station manufacturers and types (including AC, DC). They also allow a whole range of third-party systems to be incorporated and can be flexibly upgraded and expanded. They demonstrate their advantages over hub-satellite systems when there are five charging points or more.

Inherently, the three different solutions achieve different results in fulfilling the main requirements for an efficient and future-proof charging infrastructure. It is worth considering all details and eventualities—with regard to how the load management, which is recommended for charging station numbers as low as three, should be configured, for example . This is particularly important in terms of the future scalability—how is it ensured that charging stations that are to be added just two or five years later can be also integrated—in order to set up a sustainable foundation for the future.

There are two possible choices for the load management: Static load management splits a specific fixed power value across the electric cars being charged. In some cases this is sufficient when only a few cars are to be charged at the same location. Dynamic load management, on the other hand, measures the building’s or the site’s current power intake and then calculates the maximum available power for the charging infrastructure and distributes it optimally across the electric cars being charged. Dynamic load management utilizes the maximum possible power of the power connection and is considerably more efficient than the static version. This is particularly beneficial at night, when cars are generally parked, as much more power is then available for charging.

Hub-satellite or open system?

In general, both hub-satellite and open systems are capable of simultaneously charging electric cars at one building connection without overloading it. Hub-satellite solutions are available at a comparatively inexpensive entry price but do have some disadvantages: the number of charging stations, for instance, is limited depending on the supplier, with most offering 10 to 20 charging points for this type of solution. Another disadvantage is that the hub-station is a bottleneck. If it fails, all satellite stations will also fail, and it cannot be guaranteed that the satellite charging points will start up reliably when the hub wallbox is restarted.  Furthermore, tests such as the one carried out by the ADAC (Europe’s largest motoring association) have shown that some hub-satellite systems have problems with controlling unbalanced load, resulting in the mains connection not being fully utilized and power reserves being wasted. According to the ADAC, hub-satellite solutions are not always able to correctly process the communication signals of third-party systems, such as energy management systems, and some manufacturers charge an additional fee for individual options like remote access, user management, and reporting. Experience has also shown that for future expansion stages there is a risk of stranded assets, as you are generally limited to one single manufacturer. 

Open systems, with their intelligence located in a central DIN rail controller in the fuse box/switch cabinet, do entail slightly higher costs for the initial installation. However, they also have high savings potential during operation and for future expansion stages, and have certain advantages that no other solution can offer. An open system has a very low risk of failure, for example, as when one single charging point fails, this does not prevent all the other charging stations from continuing to function independently. Open systems can also integrate a practically unlimited number of charging stations. Thanks to the use of the established communication protocol OCPP, both DC and AC stations and various manufacturers can be combined. Furthermore individual charging points can be prioritized, for example for vehicles belonging to management or those that are used frequently. Having one or more quick DC stations to supplement the classic AC stations brings clear advantages: this enables, for example, rapid charging when power is needed urgently or, if the stations are publicly accessible, they can also be made available for public use, for a fee. 

Open systems do not pose any risk of stranded assets. On the contrary, they can be expanded flexibly at any later date, regardless of the manufacturer. This also means that the charging infrastructure can continually grow, for example if a company fleet is to be converted to electric cars gradually over a prolonged period. 

Hub-satellite systemsOpen systems
+ Price: comparatively low entry price- Price: comparatively higher purchase price
- High risk of failure: hub station is bottleneck; failure of hub station also affects satellite stations+ Low risk of failure: charging stations function independently even if one station
- Signal problems: signals from energy management or charging management third party systems can cause problems + Third-party system integration: open systems can interact with third-party systems using standard protocols such as Modbus or OCPP 
- Limited number of charging stations: hub-satellite setup is limited to the number of connected charging points (currently mostly around 10-20 can be integrated)+ High number of charging stations: almost unlimited number of charging stations can also be integrated successively
- No different charging stations: typically only one type of charging station can be integrated into a hub/satellite setup; risk of stranded assets+ Different charging station manufacturers and types: combination of DC and AC stations as well as different manufacturers are possible 
Recommended use: smaller installations that are set up in one go with one manufacturer and do not have to meet any major requirements (fail-safety, interfaces, etc.) Recommended use: larger installations that want to combine several manufacturers or different types of charging stations (AC/DC) and also value a modular, future-proof system

Given the wide range of different systems, to find the perfect charging solution for your individual usage scenario, you should intensively look into your current and future needs regarding equipment and function before the system is set up. It is also advisable to find support from experienced experts for the product selection, planning of the installation, and configuration of the system. Then nothing stands in the way of switching to electric mobility successfully. 

For all topics related to smart charging and energy solutions, contact our team of experts - we will be happy to advise you.

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