Photovoltaics

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Updated – September 5, 2023

Legal situation as of January 1st, 2021

The new EEG (law for the expansion of renewable energies) has come into force since January 1st, 2021, including for photovoltaic systems.

Accordingly, self-consumption from old and new photovoltaic systems up to and including 30 kWp is exempt from the EEG surcharge. For self-consumption of up to 7 kWp, no SmartMeters need to be installed for data collection.

Compensation for electricity fed in

The feed-in tariff decreases as the output increases and the date of initial commissioning increases. This means that the more the system delivers and the later it is installed, the lower the remuneration.

Systems put into operation on September 1st, 2021 will be reimbursed with 0.0725 euros for up to 10 kWp. At 0.30 .. 0.35 euros per kWh for electricity purchased, the remuneration represents approximately 20% of the price to be paid for electricity purchase. Everyone can decide for themselves whether feeding in is worthwhile.

The complete one might make more sense ownconsumption be.

Selection of PV modules

While PV modules used to be many times more expensive and far less efficient, the variety of designs, performances, efficiencies, etc. available today is huge.

Finding the happy medium is not easy, but it is possible.

You choose a well-known manufacturer, a design that uses just a few modules, offers the highest possible efficiency and comes as close as possible to the desired performance.

The design is usually rectangular, but differs in the width and length of the modules. The available area is therefore the limiting factor when choosing the module size. If you build several rows, you have to take into account the shading of the modules behind the first row. You have to plan the appropriate distance between the rows of modules so that no shadows are cast on the next row when the sun is at a lower position. This minimizes the usable area.

Determining the required distances can be done using software or, more easily, using drawings.

The data required for this is:

• geogr. Length/width of installation location
• Azimuth of the sun at the winter solstice 12:00 p.m
• Elevation height of the top edge of the module
• Elevation angle (usually 30°)

This work by. provides good documentation on this Dr.-Ing. Volker Quaschning and Prof. Dr.-Ing. habil. Rolf Hanitsch. As is usually the case, achieving an optimal result always involves effort, be it just in terms of familiarizing yourself with the theoretical considerations that form the basis for it.

Once the given parameters have been determined, it is now important to find the appropriate modules. For a maximum of 7 kW, 20 modules of 350W each are available, which can be connected in 2 strings of 10 modules each.
A string refers to a series connection of modules. With an example module MPP (Maximum Power Point) voltage of 35 V, this results in an inverter input voltage of 350 V DC.

The number of possible strings depends on the choice of inverter. Small inverters only have one MPP tracker and therefore only allow a single string to be connected.

Small inverter 1×1

There are basically three types of inverters: those that run on or off the grid, or a combination of both operating modes.

Mains led Inverters synchronize with the 50 Hz grid frequency. If this fails during a power outage, they will stop working for the duration of the power outage.

Off-grid Inverters are used in so-called island systems, i.e. independent PV systems that are not connected to the public grid.

Inverters that combine both operating modes are called Hybrid-inverter. On the one hand, they are connected to the public network, but also, via so-called MPPT controllers, to batteries that are charged by the PV modules that are also connected when the sun shines. In the event of a grid failure, the hybrid inverter simply switches to battery operation and thus maintains the power supply within its load limits. When the public network is available again, it switches to battery charging.

Inverter technical details

A three-phase device can be chosen as an inverter, for example a Kostal Piko IQ 8.5 for just under 2,000 euros. Single-phase devices are only available up to 5 kW, such as the Kostal Piko MP plus 5.0-2 for around 1,200 euros.

Both devices have two MPP trackers each (connection of two strings).

Which data is relevant for choosing an inverter?

Entry phases

Smaller inverters up to approx. 5 kW usually offer one phase, larger ones three phases. For the connection on the mains side, a three-wire cable is required for a single-phase connection and a five-wire cable for a three-phase connection.

AC rated power

The AC nominal line refers to the maximum power of the inverter and therefore also the highest PV power that can be installed. This means that no more PV modules may be connected to an inverter than its rated AC output.

Number of MPP trackers

The number of MPP trackers available in an inverter determines the number of strings that can be connected. Smaller inverters usually only have one MPP tracker, larger ones have two or more. The more MPP trackers are installed, the more expensive the inverters are.

MPP area

The DC voltage range specified here defines the maximum possible DC input voltage on the MPP Tracker. The added DC voltage of all modules combined in series in a string must not exceed the upper limit. For example, if the range is specified as 360 .. 600 V, the MPP voltage for the PV module is described as 35V, resulting in 600 / 35 = 17.14, so a maximum of 17 modules in series as one string are possible.

Input current

The input current indicates the sum of all string currents, which results from adding the MPP currents of a module of each string. If a PV module has an MPP current of 9.92 A and two strings are installed, 9.92 + 9.92 results in an input current of 19.84 A.
Assuming the inverter can only handle a maximum of 16 A, then modules with an MPP current of a maximum of 16 – 9.92 = 6.08 A must be installed in the second string.

Open circuit voltage

The open-circuit voltage of PV modules is always above the MPP voltage. It is higher at low outside temperatures and lower at high temperatures. If the inverter has a maximum no-load voltage of, for example, 750 V, the module no-load voltage must not exceed 750 / 17 = 44 V with 17 installed modules.

Efficiency

Efficiency describes the efficiency with which an inverter converts the incoming direct voltage into outgoing alternating voltage. However, since the efficiency varies as the intensity of the sun fluctuates, knowledge of the voltage dependency is important for a correct design. As a rule, lower voltage goes hand in hand with higher efficiency.
In this context, maximizing the upper limit voltage of the MPP range is less efficient. The configuration with several strings is therefore better, although this means a higher financial outlay when purchasing the inverter.