Cell is supplied with electricity by photovoltaic system, which it was supplied by GORENJE SOLAR. It is installed on the roof of the residential unit.

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A photovoltaic system consists of six modules with polycrystalline solar cells and total area of 9.6 m², six lead batteries with liquid electrolyte, DC / AC inverter that converts direct current into a alternating current and electronic components that control charging and discharging of batteries.

Each of the six solar cell modules has dimensions of 1663 x 998 mm, consisting of 60 in a series connected polycrystalline silicon solar cells. Basic electro-technical module properties are: peak power 220 Wp (at solar radiation of 1000 W/m², cell temperature of 25°C and solar spectrum AM 1.5), short circuit current Isc is 8.25 A, the open circuit voltage Voc is 37.2 V and maximum electrical power at Impp 7.55 A and Vmpp 29.1 V (peak power equals Impp x Vmpp ~ 220 Wp). Modules are resistant to hail impact with grain size of 25 mm with speed at the impact of 23 m/s.

Photovoltaic system has two electrically separated circuits, each with three modules connected in series. Thus, the voltage in the circuit is increased (maximum open circuit voltage is 112 V), while the electric current remains the same. Thus electrical cables can have a smaller cross section and are cheaper.

Electricity is stored in six TAB 190P lead batteries with liquid electrolyte. Total battery bank capacity is 1350 Ah, battery voltage is 12V. Taking into account the 75% depth of discharge, which corresponds to ~ 300 cycles of charging/discharging in a life time, amount of stored electricity in batteries is 13.5 kWh. Due to liquid electrolyte, batteries are inserted in the stainless steel container, which retains the electrolyte in case of battery cells damage.

Each circuit of the PV modules is connected to batteries through charge controller. Controllers have built-in MPPT function (maximum power point tracker) that adjusts the PV modules performance to always operate in the point of maximum electric power (Impp, Vmpp). Controllers ensure correct battery charging and supply of 24 V DC network. Directly connected to the batteries is DC/AC inverter (24 V DC/240 V AC) with maximum sustained power of 1000 W. The inverter is connected to a 220 V power lines in the Cell.

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What is the life-time of the PV modules? A: With a certificate a manufacturer states that after 25 years of PV system operation peak electrical power will be at least 80% of the today’s. Therefore the PV system will have a peak power of 1.06 kWp in the year 2037!

How much electricity is stored in batteries of the Cell? A: With regard to the allowed batteries depth of charge, 13.5 kWh of electricity is stored. For comparison: the Toyota Prius+ is capable of storing 5.2 kWh, the Nissan Leaf, which is electric car, 24 kWh of electricity. Considering the average household consumption in Slovenia of 10 kWh of electricity per day, we expect that stored electrical energy in the Cell is sufficient for one week residency of one person who uses electricity wisely. SmartMonitor will warn the resident (more in: >monitoring >SmartMonitor).

Why do you use lead- batteries in the Cell? A: Because of its price price ~ 80 €/kWh. Remark: Lead batteries have a few comparative advantages, such as low self-discharging and possibility of deep discharge, but they are heavy. In the Cell we store 36 Wh in one 1 kg of battery. As an alternative, the nickel-hydride (65 Wh/kg) and lithium-ion (170 Wh/kg) battery are used. But these are too expensive for PV systems (500 €/kWh). By 2020 the technology of lithium-sulphur (300 Wh/kg) and by 2025 lithium-air (1000 Wh/kg) of batteries should be developed.