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Customer application 25 January 2022
The Self-Sufficient Kindergarten

The Heiligkreuz kindergarten in Würzburg, Bavaria, is designed for the future – it is where tomorrow’s adults learn and play. And the power supply also offers insight into an aspect of our energy landscape that is becoming more and more common: properties that produce their own electricity. A photovoltaic system on the roof handles that. A storage device, which holds excess power to be used later as needed, makes the system economical.

In 2017, the kindergarten of the Heiligkreuz Catholic Church Foundation, built in 1932, had to be replaced with a new building. The power supply also had to be completely redesigned, because with the expansion, the existing connection, which had a maximum capacity of 72 kW, was no longer sufficient. The updated equipment, including a kitchen, elevators and a decentralized hot water supply system, required significantly more power.

For example, the plan involved tankless water heaters, which have a relatively high heating capacity. They also provide a high level of hygiene because they heat the water as it flows through and are installed close to where it is used. This guarantees a precisely adjustable temperature and prevents growth of legionella bacteria.

However, because the energy consumption doubles, this system sometimes requires more electricity than the existing connection can supply. The obvious option would have been to expand the capacity at the mains connection. However, the grid operator would have had to build a new branch point from the public grid and lay more underground cable. An additional substation would also have been necessary.

So the client decided against the conventional solution. Instead, they chose a solution that just a few years ago sounded futuristic, but is now widely used: operating a decentralized, renewable energy system to mostly cover their own peak loads.

Controlling Photovoltaics and Energy Storage Systems – Here’s How WAGO Supports You:

  • The PFC200 telecontroller handles the exchange of complex information among all the components.

  • The energy meter installed at the mains supply point is connected via the RS-485 onboard interface.

  • VPN encryption also ensures maximum security for access via the Internet.

A Daycare Center as a Prosumer

Private households and small businesses accomplish this primarily through photovoltaics. The goal is no longer to feed energy into the grid, but rather to cover their own energy consumption to the greatest possible degree. However, the systems are only economical if they are capable of storing surplus electricity. The reason is that solar energy fluctuates, independent of when most power is needed. This is another area where solar energy has profited from market developments – because the costs for systems based on lithium-ion batteries have also fallen.

The kindergarten became a prosumer as well: It is simultaneously a producer and a consumer and has become as independent of the power company as possible. A photovoltaic array has been in operation on the roof of the new daycare center since late 2017. With a peak output of 60 kilowatts, the system would not be sufficient on its own to cover the peak loads, which exceed the maximum connection output. Therefore, TEAM ELEKTRO BECK, headquartered in Würzburg, installed an additional stationary electrical storage system, which the company developed jointly with BMW, next to the solar array.

Storage Devices Limit Mains Power Consumption

The electrical storage system consists of a BECK power and control unit and six new BMW i3 lithium-ion batteries with a nominal storage capacity of 33 kilowatt-hours each. This equals a total net capacity of 175 kilowatt-hours. This storage buffer allows demand-driven utilization of the electricity generated on the roof.

In Würzburg, the system is designed to limit the output and minimize mains power consumption. The power the system generates is the first to be used. If it is not sufficient, power is drawn from the lithium-ion batteries. Priority is given to storing the self-produced electricity, which cannot be consumed immediately in low-demand periods.

“In addition to optimizing consumption by the owner, it’s possible to avoid higher network charges for supplying power by discharging the storage device when peak loads are imminent and charging it during low-load periods,” explains Ralf Klingenberg from BECK AUTOMATION. “Behind all this, there’s an algorithm that continuously calculates the optimal battery output. This also prevents unnecessary battery discharge.”

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The six lithium-ion batteries of the energy storage system are located in the basement of the daycare center.

In addition, load management can be used to lock individual devices according to a priority list, such as the tankless heater that provides the cleaning water. If the generation from the PV array is insufficient, mains power can be used to charge the batteries during off-peak hours – for instance, overnight. If, however, the power generated by the PV system exceeds the consumption, and the batteries are already full, then surplus energy can be fed into the grid as well.

Optimizing consumption by the owner, discharging the storage device when load peaks are imminent and charging it when loads are low help avoid higher grid charges for providing power.

Ralf Klingenberg, BECK AUTOMATION

Complex Communication

BECK AUTOMATION used WAGO components to implement the complex information exchange among storage devices, the grid, the solar array and the loads. A PFC200 telecontroller from the WAGO I/O SYSTEM 750 functions as the control and communication unit. The onboard RS-485 interface communicates with the energy meter installed at the mains supply point using the open-source Modbus RTU protocol. The recorded energy measurement data can be used to control energy generation and storage efficiently.

The controller connects to the Internet via an encrypted Virtual Private Network (VPN), which meets the highest security standards. In addition, the WAGO PLC transmits live data – for example, on the self-generated energy, the battery charging states or the consumption data – to the BECK server and sends historical data to the BECK database for evaluation. This allows further optimization of the interplay between the solar and battery parts of the combination.

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The PLC coordinates the energy flows between the solar array, storage device, grid and loads.

Text: Andreas Siegert | WAGO

Photo: Jilek | Bildschön

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