Office ZEV under DC current

A project by Marti Technik AG

A solar roof with many modules as well as different orientations and everything is brought together on one inverter. This is only possible with AMPTs and a small DC microgrid. Highly efficient due to low conversion losses and optimised for self-consumption with the salimax® salt battery storage system from innovenergy®.

If Mr. Marbach drives to the office in the morning, he charges his car downstairs in the garage at one of the seven e-charging stations. He enters his office, switches on the light and starts up his computer. He knows exactly where his electricity comes from – from the roof of the house. And his electricity bill no longer comes from the grid operator, but from the office next door.

The office and commercial building of IL Immobiliengesellschaft Lagerhausweg AG in Bern-Bümplitz houses several companies that are all united in a consumer association (ZEV). There is only one electricity bill from the grid operator to the entire building. The individual consumption of the companies is billed to each other via internal consumption meters.

The building's annual electricity consumption is 550,000 kilowatt hours. Approximately 25 percent of this consumption is covered by the photovoltaic system with 146 kilowatt peak on the roof. The energy from different module orientations is brought via AMPTs to a DC link with a constant 750 volts. From there, the power goes directly to the DC bus of the salimax® salt battery storage system with 45 kilowatt hours of capacity and an integrated high-efficiency inverter, consisting of three elements, with 75 as well as another inverter with 50 kilovolt amperes of power. From here, the energy flows as alternating current into the internal AC grid.

Why AMPT and DC Link

On a larger and more complicated solar roof, the different orientations of the modules normally require their own MPPT input on the photovoltaic inverter. However, the number of MPPT inputs is limited. Therefore, in string planning with common photovoltaic inverters, depending on the size and complexity of the system, the solar planner has to compromise for optimal tracking of all strings. With the AMPT String Optimisers, these strings can be bundled more easily and with less loss. The number of strings is reduced by half thanks to the AMPT String Optimisers, which also means less installation work.

The energy from the AMPTs is routed on a DC link, a DC cable, with a constant voltage of selectable 400 to 850 volts to one or more inverters to the internal alternating current (AC) grid. The inverters in the salimax®, with outputs of up to 400 kilowatts, operate with constant direct voltage (DC) and an energy efficiency of almost 98 percent. The electricity from the photovoltaic system can therefore be used to the maximum and without compromise.

Electric car charging stations with DC/DC converters can also be connected directly to the DC Link. With bidirectional DC/DC converters, even the batteries of the vehicles at the e-charging stations can be used as external mobile energy storage units and integrated into the intelligent load management system.

More than just emergency power

Self-sufficiency is an important argument for many companies – to be prepared for emergencies, for power failures in an increasingly uncertain public electricity grid. People talk about emergency power. But most people confuse emergency power capability with off-grid capability. If the mains power actually fails, everything is switched off and the battery system steps in and supplies emergency power until it is empty. In the best case, this lasts a few hours to a day, depending on the storage capacity of the battery system and the consumption of the enterprise. But this is not true self-sufficiency, it is merely a short stopgap. As a rule, this is also enough until the public grid supplies again and everything resumes its usual operation.

Off-grid capability goes beyond the emergency power state. In this case, the photovoltaic system continues to operate normally or partially. Solar modules and battery continue to supply consumers within its own grid - the company is its own island with energy production and consumption. This is possible to the full extent due to the company's own DC grid with DC link and a DC-coupled storage system.

Ecology was the decisive reason

The customer wanted an environmentally friendly, recyclable and economical battery storage system out of conviction, which does not require any special construction measures, is easy to maintain and, above all, safe.

Marti Technik AG specialises in the assumption and execution of overall planning and engineering work, the fabrication and production of complete plant systems in the areas of materials handling technology, plant construction, mechanical and building engineering, switchgear combinations, photovoltaics and energy storage including eMobility.

Thomas Graber 
Head of Plant Engineering

Marti Technik AG