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Czech Sustainable House – Off-grid Use Case

Realization: 2020

Location: Cesky Krumlov, Czech Rep.

Implemented services: In cooperation with GWL and Asolar, installed Amvolt Battery Pack 21 kWh, complete system of Victron Energy inverters, solar power plant 15 kWp

Costs: Amvolt Battery Pack 21.1 kWh, price: 18,103 EUR w/o VAT (solar panels and inverters not included)

The energy source for the Czech Sustainable House (Český soběstačný dům in Czech) is a large photovoltaic power plant with an installed capacity of 15 kWp, which is supplemented by a 21 kWh Amvolt Battery Pack.

Photovoltaics are sized this way especially for autumn and winter. Thanks to its size, it is able to supply several hundred watts even in fog and low cloud conditions and thus cover the basic consumption of the house. At this output, there is no need to install a larger battery for off-grid operation - the times when a PV system this large would give nothing at all do not last long.

The power plant was built in cooperation between GWL, Amvolt and Asolar directly to the specifications of the Czech Sustainable House. The aim was to find a technical solution that would be as safe as possible, reliable in the long term and easy to repair in the event of a breakdown. The power plant is therefore not dependent on the technology of one manufacturer, all components are interchangeable with others with similar parameters.

The fitted electronics are very robust and oversized, duplicated or triplicated to ensure that the failure of one or more components does not compromise the power supply. At the same time, it is deliberately not too sophisticated according to the motto "the simpler it is, the less it goes wrong and the easier it is to fix".

The power plant consists of 48 monocrystalline panels with an output of 330 Wp in an all-black design. The roof is free of irregularities that could cause shading and the panels are evenly illuminated during the day. This eliminates the need for optimizers, further simplifying the power plant.

The electricity from the panels is fed to three controllers, from which it continues to the Amvolt Battery Pack and from there to the inverters and appliances in the house. Professionally, this architecture is called "DC coupling". If the house's consumption is met and there is still a surplus of electricity, the batteries start to recharge, quite naturally, based on Kirchoff's laws, without the need to control these energy flows. If the consumption is met and the batteries are charged, the electricity goes from the roof via the controllers straight to the three heating rods in the storage tank. And when all the consumption and storage are satisfied in summer, the power station stops pulling electricity from the panels and they just continue to heat up the roof like a normal roof covering.

Charging of the batteries and heating of the storage tank is done by DC current without the inverter, thus saving electricity and the inverter. With three controllers, power is also provided to the house even if one of them breaks down.

For the same reason, the plant contains three inverters – main, backup and emergency. In addition to a backup in case one of them fails, switching between the inverters allows for even more efficient use of electricity in the house. The main inverter provides 8,000 watts (20,000 watts peak) of power when there is plenty of electricity from the panels – especially in the summer. The high power output can be used for charging an electric car, for example. But at the same time, the main inverter has a peak self-consumption of 80–120 W, which would continually drain valuable electricity in winter.

Therefore, for the winter period, the plant switches to a backup inverter, which is limited to 4,000 W (10,000 W peak) but also has a lower self-consumption of 30–50 W. Finally, there is a third emergency inverter with a capacity of 1,300 W (2,800 W peak) and a self-consumption of 5 W, which provides power to the basic technologies in the house, lighting and several sockets.

The installed Amvolt Battery Pack consists of 16 LFP cells with a total capacity of 21 kWh, which is electronically limited to 18 kilowatt hours of usable power. This is to extend battery life, but full capacity can be accessed by software if necessary. The battery is monitored by the Elerix CPM (Cell Performance Monitor battery management system (BMS), which monitors the voltage of each cell to ensure that it does not fall outside the set values. If this happens, the system alerts the user and if the user does not respond and it is necessary, the battery disconnects itself. The voltage of this Amvolt Battery Pack is a safe 51–55 volts.

More information about the Czech Sustainable House project at www.csdum.cz/en.

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