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Battery
We store energy storage for storing surplus PV production, to ensure energy in the event of a grid failure, etc., using a stationary battery.
Battery capacity for this purpose is based on several factors:
Required backup time in case of grid failure. An example of calculating the appropriate capacity for the required backup time and estimating the backed up appliances is given in the following chapter.
Suitable capacity with regard to the performance of PV panels. For efficient use of energy from PV panels, it is necessary to appropriately dimension the battery capacity, one of the stated possible values is 1.75 kWh of battery per 1 kWp of PV panel power.
Another very important factor is the price of the battery, its life and the number of cycles - here is the key technology of the battery itself.
Technology
Due to the service life, number of cycles, operational safety, LiFePO4 technology is currently doing very well. All other information, connections and examples in the following text assuming the use of cells for battery assembly are given for LiFePO4 technology. These batteries have a lifespan of up to 20 years, 5000 to 8000 cycles (depending on the depth of discharge, charging and discharging speed, reaching the number of cycles does not destroy the battery, but a decrease in capacity to eg 80%), they do not mind operation in negative temperatures (can be placed in garage or other non-tempered spaces), do not require special storage from the point of view of safety (it is possible to place them in living spaces). On the contrary, they are very sensitive to overcharging or complete discharge, so the so-called BMS is necessary for their efficient operation - see next text.
BMS
A battery composed of cells based on LiFePO4 technology requires monitoring and balancing of each cell, the so-called BMS (battery management system), for reliable operation. The BMS monitors the maximum and minimum voltage and maximum temperature for each cell, thus ensuring that the cell is not destroyed by overcharging or complete discharge.
MPPT charger.
A charger with the MPPT (Maximum Power Point Tracking) function is used for optimal use of the energy of the photovoltaic panels. The charger with this function contains a DC / DC converter that maintains the maximum value of the power that the PV modules are able to supply for charging the connected battery (or other use in the system). The integrated and conventional inverters have the same function.
Standard inverter (standard PV plant without batteries)
For common installations without batteries, we use standard grid inverters that convert the DC input voltage from the connected PV modules to 230 VAC. They are equipped with the MPPT function (they include an MPP tracker), smaller outputs (approx. 1.5 kW to 3 kW) usually have the option of connecting one string of PV panels, larger inverters (3 to 5 kW) then 2 strings.
The output of a standard inverter is indicated by the maximum number of PV modules that can be connected to its input. Due to the legislation and common installations in the family house, the output of the inverters ranges from approx. 2 kW to 5 kW in a single-phase design. For larger outputs and three-phase versions, a number of inverters are available on the market, but in the following examples we will focus on single-phase installations in the above outputs.
The inverter's own consumption is an important parameter not only for island systems (where it can significantly reduce the amount of usable energy from batteries), but also for hybrid systems, because even here it represents an undesirable loss of energy. Smaller inverters (up to approx. 4 kW) should optimally have their own consumption of max. 40 W, larger inverters (over 5 kW) up to approx. 80 W.
The maximum efficiency of inverters is around 90 ÷ 98 %. Even more accurate is the so-called European efficiency, which better takes into account the nature of sunlight, the best inverters reach up to 97 %.
PV panels
Depending on the technology, we can choose between monocrystalline, polycrystalline and amorphous panels.
Amorphous panels have the advantage over the others of the smaller effect of partial shading of a string panel (chimney, trees, poles, etc.), but they have less efficiency and are as a result more expensive.
Monocrystalline panels have better efficiency in accurate orientation to the sun and direct light, while polycrystalline panels work better in poorer orientation and clouds, so for common stationary applications on the roof of the house there is not much difference between them.
Example of a PV panel:
Axitec AC-250P panel, polycrystalline panel, parameters at 1000 W / m2, temperature 25 ° C:
Nominal power 250 Wp
Nominal output voltage 31.45 V
Rated current 7.98 A
Efficiency 15.37%
Location of panels.
The optimal slope for the year-long largest gain is an angle of about 35 °, orientation to the south. An inclination of up to 45 ° provides only slightly worse gains in the order of %.
In the case of using 2 strings, it may be advantageous to place one string in the orientation of SW, the other SE - we get a more even distribution of power during the day.
Power reduction by heating the panel. The normal value of the temperature coefficient of the panel is 0.47 %/°C, so in summer the performance of the panels may decrease by more than 10 %, on the contrary, in winter the performance increases (the manufacturer typically states performance parameters at 25 ° C).